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<TITLE>7. Basis sets</TITLE>
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<B> Next:</B> <A NAME="tex2html1108"
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HREF="node10.html">8. Effective Core Potentials</A>
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HREF="node8.html">6. Geometries</A>
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HREF="node2.html">Contents</A></B>
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<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>
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<UL>
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<LI><A NAME="tex2html1109"
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HREF="node9.html#SECTION00910000000000000000">7.1 Basis set library</A>
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<LI><A NAME="tex2html1110"
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HREF="node9.html#SECTION00920000000000000000">7.2 Explicit basis set definition</A>
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<LI><A NAME="tex2html1111"
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HREF="node9.html#SECTION00930000000000000000">7.3 Combinations of library and explicit basis set input</A>
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</UL>
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<!--End of Table of Child-Links-->
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<HR>
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<H1><A NAME="SECTION00900000000000000000">
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7. Basis sets</A>
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</H1>
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<A NAME="sec:basis"></A>
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<P>
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NWChem currently supports basis sets consisting of generally
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contracted<A NAME="tex2html13"
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HREF="footnode.html#foot1793"><SUP>7.1</SUP></A> Cartesian Gaussian functions up to a maximum angular
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momentum of six (<IMG
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WIDTH="13" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
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SRC="img59.gif"
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ALT="$h$"> functions), and also <IMG
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WIDTH="20" HEIGHT="28" ALIGN="MIDDLE" BORDER="0"
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SRC="img60.gif"
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ALT="$sp$"> (or L)
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functions<A NAME="tex2html14"
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HREF="footnode.html#foot1794"><SUP>7.2</SUP></A> . The <TT>BASIS</TT> directive is used to
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define these, and also to specify use of an effective core potential
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(ECP) that is associated with a basis set; see Section <A HREF="node10.html#sec:ecp">8</A>.
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<P>
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The basis functions to be used for a given calculation can be drawn
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from a standard set in the EMSL basis set library that is included in
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the release of NWChem (See Appendix <A HREF="node41.html#sec:knownbasis">A</A> for a list
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of the standard basis sets currently supplied with the release of the
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code). Alternatively, the user can specify particular functions
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explicitly in the input, to define a particular basis set.
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<P>
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The general form of the <code>BASIS</code> directive is as follows:
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<P>
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<PRE>
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BASIS [<string name default "ao basis">] \
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[(spherical || cartesian) default cartesian] \
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[(segment || nosegment) default segment] \
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[(print || noprint) default print]
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[rel]
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<string tag> library [<string tag_in_lib>] \
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<string standard_set> [file <filename>] \
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[except <string tag list>] [rel]
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...
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<string tag> <string shell_type> [rel]
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<real exponent> <real list_of_coefficients>
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...
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END
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</PRE>
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<P>
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Examining the keywords on the first line of the <code>BASIS</code> directive:
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<P>
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<UL>
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<LI><TT>name</TT>
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<P>
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By default, the basis set is stored in the database with the name
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<code>"ao basis"</code>. Another name may be specified in the <code>BASIS</code>
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directive, thus, multiple basis sets may be stored simultaneously in the
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database. Also, the DFT (Section <A HREF="node13.html#sec:dft">11</A>)
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and RI-MP2 (Section <A HREF="node18.html#sec:rimp2">16</A>) modules and the
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Dyall-modified-Dirac relativistic method (Section <A HREF="node11.html#sec:dyall-mod-dir">9.2</A>)
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require multiple basis sets with specific names.
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<P>
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The user can associate the <code>"ao basis"</code> with another named basis
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using the <code>SET</code> directive (see Section <A HREF="node7.html#sec:set">5.7</A>).
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<P>
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</LI>
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<LI><TT>SPHERICAL</TT> or <TT>CARTESIAN</TT>
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<P>
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The keywords <code>spherical</code> and <code>cartesian</code> offer the option of
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using either spherical-harmonic (5 d, 7 f, 9 g, ...) or Cartesian
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(6 d, 10 f, 15 g, ...) angular functions. The default is
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Cartesian.
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<P>
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Note that the correlation-consistent basis sets were designed using
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spherical harmonics and to use these, the <code>spherical</code> keyword
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should be present in the <code>BASIS</code> directive. The use of spherical
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functions also helps eliminate problems with linear dependence.
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<P>
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</LI>
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<LI><TT>SEGMENT</TT> or <TT>NOSEGMENT</TT>
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<P>
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By default, NWChem forces all basis sets to be segmented,
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even if they are input with general contractions or <IMG
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WIDTH="15" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img63.gif"
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ALT="$L$"> or sp
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shells. This is because the current derivative integral program cannot
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handle general contractions. If a calculation is
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computing energies only, a
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performance gain can result from exploiting generally contracted basis
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sets, in which case <TT>NOSEGMENT</TT> should be specified.
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<P>
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</LI>
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<LI><TT>PRINT</TT> or <TT>NOPRINT</TT>
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<P>
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The default is for the input module to print all basis sets encountered.
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Specifying the keyword <code>noprint</code> allows the user to suppress this output.
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<P>
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</LI>
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<LI><TT>REL</TT>
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<P>
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This keyword marks the entire basis as a relativistic basis for the purposes
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of the Dyall-modified-Dirac relativistic integral code. The marking of the
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basis set is necessary for the code to make the proper association between
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the relativistic shells in the ao basis and the shells in the large and/or
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small component basis. This is only necessary for basis sets which are to be
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used as the ao basis. The user is referred to Section <A HREF="node11.html#sec:dyall-mod-dir">9.2</A>
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for more details.
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<P>
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</LI>
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</UL>
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<P>
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Basis sets are associated with centers by using the tag of a center in
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a geometry that has either been input by the user (Section
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<A HREF="node8.html#sec:geom">6</A>) or is available elsewhere. Each atom or center with
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the same <code>tag</code> will have the same basis set. All atoms must have
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basis functions assigned to them -- only dummy centers (X or Bq) may have no
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basis functions. To facilitate the specification of the geometry and
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the basis set for any chemical system, the matching process of a basis
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set tag to a geometry tag first looks for an exact match. If no match
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is found, NWChem will attempt to match, ignoring case, the name or
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symbol of the element. E.g., all hydrogen atoms in a system could be
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labeled ``H1'', ``H2'', ..., in the geometry but only
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one basis set specification for ``H'' or ``hydrogen'' is necessary.
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If desired, a special basis may be added to one or more centers (e.g.,
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``H1'') by providing a basis for that tag.
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If the matching mechanism fails then NWChem stops with an appropriate
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error message.
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<P>
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A special set of tags, ``*'' and tags ending with a ``*'' (E.g. ``H*'')
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can be used in combination with the keyword <code>library</code> (see section
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below). These tags facilitate the definition of a certain type of basis
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set of all atoms, or a group of atoms, in a geometry using only a single
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or very few basis set entries. The ``*'' tag will not place basis sets
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on dummy atoms, Bq* can be used for that if necessary.
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<P>
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Examined next is how to reference standard basis sets in the basis set
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library, and finally, how to define a basis set using exponents and
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coefficients.
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<P>
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<H1><A NAME="SECTION00910000000000000000">
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7.1 Basis set library</A>
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</H1>
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<P>
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The keyword <code>library</code> associated with each specific <code>tag</code>
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entry specifies that the calculation will use the standard basis set
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in NWChem for that center. The variable <code><standard_set></code> is the
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name that identifies the functions in the library. The names of
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standard basis sets are not case sensitive. See Appendix
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<A HREF="node41.html#sec:knownbasis">A</A> for a complete list of the basis sets in the
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NWChem library and their specifications.
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<P>
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The general form of the input line requesting basis sets from the NWChem
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basis set library is:
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<PRE>
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<string tag> library [<string tag_in_lib>] \
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<string standard set> [file < filename> \
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[except <string tag list>] [rel]
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...
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</PRE>
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<P>
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For example, the NWChem basis set library contains the Dunning cc-pvdz
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basis set. These may be used as follows
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<PRE>
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basis
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oxygen library cc-pvdz
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hydrogen library cc-pvdz
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end
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</PRE>
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<P>
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A default path of the NWChem basis set libraries is provided on installation
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of the code, but a different path can be defined by specifying the keyword
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<code>file</code>, and one can explicitly name the file to be accessed
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for the basis functions. For example,
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<PRE>
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basis
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o library 3-21g file /usr/d3g681/nwchem/library
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si library 6-31g file /usr/d3g681/nwchem/libraries/
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end
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</PRE>
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This directive tells the code to use the basis set <code>3-21g</code> in
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the file <TT>/usr/d3g681/nwchem/library</TT> for atom <code>o</code> and
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to use the basis set <code>6-31g</code> in the directory
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<TT>/usr/d3g681/nwchem/libraries/</TT> for atom <code>si</code>, rather
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than look for them in the default libraries. When a directory is defined
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the code will search for the basis set in a file with the name <TT>6-31g</TT>.
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<P>
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The ``*'' tag can be used to efficiently define basis set input directives
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for large numbers of atoms. An example is:
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<PRE>
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basis
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* library 3-21g
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end
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</PRE>
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This directive tells the code to assign the basis sets <code>3-21g</code> to
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all the atom tags defined in the geometry. If one wants to place a
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different basis set on one of the atoms defined in the geometry, the
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following directive can be used:
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<PRE>
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basis
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* library 3-21g except H
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end
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</PRE>
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This directive tells the code to assign the basis sets <code>3-21g</code> to
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all the atoms in the geometry, except the hydrogen atoms. Remember that
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the user will have to explicitly define the hydrogen basis set in this
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directive! One may also define tags that end with a ``*'':
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<PRE>
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basis
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oxy* library 3-21g
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end
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</PRE>
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This directive tells the code to assign the basis sets <code>3-21g</code> to
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all atom tags in the geometry that start with ``oxy''.
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<P>
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If standard basis sets are to be placed upon a dummy center, the
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variable <code><tag_in_lib></code> must also be entered on this line, to
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identify the correct atom type to use from the basis function library
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(see the ghost atom example in Section <A HREF="node7.html#sec:set">5.7</A> and below). For
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example: To specify the cc-pvdz basis for a calculation on the water
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monomer in the dimer basis, where the dummy oxygen and dummy hydrogen
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centers have been identified as <code>bqo</code> and <code>bqh</code>
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respectively, the <code>BASIS</code> directive is as follows:
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<P>
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<PRE>
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basis
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o library cc-pvdz
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h library cc-pvdz
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bqo library o cc-pvdz
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bqh library h cc-pvdz
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end
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</PRE>
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A special dummy center tag is <code>bq*</code>, which will assign the same basis
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set to all bq centers in the geometry. Just as with the ``*'' tag, the
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<code>except</code> list can be used to assign basis sets to unique dummy centers.
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<P>
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The library basis sets can also be marked as relativistic by adding the
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<code>rel</code> keyword to the tag line. See Section <A HREF="node11.html#sec:dyall-mod-dir">9.2</A> for
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more details. The correlation consistent basis sets have been contracted for
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relativistic effects and are included in the standard library.
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<P>
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There are also contractions in the standard library for both a point nucleus
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and a finite nucleus of Gaussian shape. These are usually distinguished by
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the suffixex <TT>_pt</TT> and <TT>_fi</TT>. It is the user's responsibility to
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ensure that the contraction matches the nuclear type specified in the
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geometry object. The specification of a finite nucleus basis set does NOT
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automatically set the nuclear type for that atom to be finite. See
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Section <A HREF="node8.html#sec:geom">6</A> for information.
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<P>
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<H1><A NAME="SECTION00920000000000000000">
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7.2 Explicit basis set definition</A>
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</H1>
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<P>
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If the basis sets in the library or available in other external files
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are not suitable for a given calculation,
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the basis set may be explicitly defined.
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A generally contracted Gaussian basis function is associated with a
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center using an input line of the following form:
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<PRE>
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<string tag> <string shell_type> [rel]
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<real exponent> <real list_of_coefficients>
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...
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</PRE>
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<P>
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The variable <code><shell_type></code> identifies the angular momentum of the
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shell, <IMG
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WIDTH="12" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img61.gif"
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ALT="$s$">, <IMG
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WIDTH="12" HEIGHT="28" ALIGN="MIDDLE" BORDER="0"
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SRC="img62.gif"
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ALT="$p$">, <IMG
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WIDTH="12" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
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SRC="img64.gif"
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ALT="$d$">, .... NWChem is configured to handle up to <IMG
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WIDTH="13" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
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SRC="img59.gif"
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ALT="$h$">
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shells. The keyword <code>rel</code> marks the shell as relativistic -- see
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Section <A HREF="node11.html#sec:dyall-mod-dir">9.2</A> for more details. Subsequent lines define
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the primitive function exponents and contraction coefficients. General
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contractions are specified by including multiple columns of coefficients.
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<P>
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The following example defines basis sets for the water molecule:
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<PRE>
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basis spherical nosegment
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oxygen s
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11720.0000 0.000710 -0.000160
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1759.0000 0.005470 -0.001263
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400.8000 0.027837 -0.006267
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113.7000 0.104800 -0.025716
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37.0300 0.283062 -0.070924
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13.2700 0.448719 -0.165411
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5.0250 0.270952 -0.116955
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1.0130 0.015458 0.557368
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0.3023 -0.002585 0.572759
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oxygen s
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0.3023 1.000000
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oxygen p
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17.7000 0.043018
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3.8540 0.228913
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1.0460 0.508728
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0.2753 0.460531
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oxygen p
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0.2753 1.000000
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oxygen d
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1.1850 1.000000
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hydrogen s
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13.0100 0.019685
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1.9620 0.137977
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0.4446 0.478148
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0.1220 0.501240
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hydrogen s
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0.1220 1.000000
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hydrogen p
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0.7270 1.000000
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oxygen s
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0.01 1.0
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hydrogen s
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0.02974 1.0
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hydrogen p
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0.141 1.0
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end
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</PRE>
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<P>
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<H1><A NAME="SECTION00930000000000000000">
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7.3 Combinations of library and explicit basis set input</A>
|
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</H1>
|
|
The user can use a mixture of library basis and explicit basis set
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input to define the basis sets used on the various atoms.
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<P>
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For example, the following <code>BASIS</code> directive augments the Dunning
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cc-pvdz basis set for the water molecule with a diffuse s-shell on
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oxygen and adds the aug-cc-pVDZ diffuse functions onto the hydrogen.
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<PRE>
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basis spherical nosegment
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oxygen library cc-pvdz
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hydrogen library cc-pvdz
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oxygen s
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0.01 1.0
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hydrogen library "aug-cc-pVDZ Diffuse"
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end
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</PRE>
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<P>
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The resulting basis set defined is identical to the one defined above
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in the explicit basis set input.
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<P>
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<HR>
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<ADDRESS>
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Edoardo Apra
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2004-05-25
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