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178 lines
7 KiB
TeX
178 lines
7 KiB
TeX
%
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% $Id$
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%
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% $Id$
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\label{sec:cosmo}
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COSMO is the continuum solvation `COnductor-like Screening MOdel'
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of A. Klamt and G. Sch\"{u}\"{u}rmann to describe dielectric screening
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effects in solvents.
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\begin{enumerate}
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\item A. Klamt and G. Sch\"{u}\"{u}rmann, J.~Chem.~Soc.~Perkin Trans.~2, 1993
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799 (1993).
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\end{enumerate}
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The NWChem COSMO module implements algorithm for calculation of the
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energy for the following methods:
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\begin{enumerate}
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\item Restricted Hartree-Fock (RHF),
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\item Restricted open-shell Hartree-Fock (ROHF),
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\item Restricted Kohn-Sham DFT (DFT),
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\item Unrestricted Kohn-Sham DFT (ODFT),
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\end{enumerate}
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by determining the solvent reaction field self-consistently
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with the solute charge distribution from the respective methods.
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Note that COSMO for unrestricted Hartree-Fock (UHF) method
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can also be performed by invoking the DFT module with appropriate
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keywords.
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Correlation energy of solvent molecules may also be evaluated at
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\begin{enumerate}
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\item MP2,
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\item CCSD,
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\item CCSD+T(CCSD),
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\item CCSD(T),
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\end{enumerate}
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levels of theory. It is cautioned,
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however, that these correlated COSMO calculations determine
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the solvent reaction field using the HF charge distribution of
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the solute rather than the charge distribution of the correlation
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theory and are not entirely self consistent in that respect.
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In other words, these calculations assume that the correlation
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effect and solvation effect are largely additive, and the combination
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effect thereof is neglected.
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COSMO for MCSCF has not been implemented yet.
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In the current implementation the code
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calculates the gas-phase energy of the system followed by the
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solution-phase energy, and returns the electrostatic contribution
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to the solvation free energy.
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At the present gradients are calculated by finite
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difference of the energy. Known problems include that the code does not
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work with spherical basis functions.
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The code does not calculate the
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non-electrostatic contributions to the free energy, except for
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the cavitation/dispersion contribution to the solvation free energy,
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which is computed and printed.
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It should be noted that one must in general take into account
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the standard state correction besides the electrostatic
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and cavitation/dispersion contribution to the solvation free energy,
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when a comparison to experimental data is made.
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Invoking the COSMO solvation model is done by specifying the input
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COSMO input block with the input options as:
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\begin{verbatim}
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cosmo
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[off]
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[dielec <real dielec default 78.4>]
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[radius <real atom1>
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<real atom2>
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. . .
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<real atomN>]
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[rsolv <real rsolv default 0.00>]
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[iscren <integer iscren default 0>]
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[minbem <integer minbem default 2>]
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[maxbem <integer maxbem default 3>]
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[ificos <integer ificos default 0>]
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[lineq <integer lineq default 1>]
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end
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\end{verbatim}
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followed by the task directive specifying the wavefunction and
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type of calculation, e.g., \verb+task scf energy+, \verb+task mp2 energy+,
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\verb+task dft optimize+, etc.
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\verb+off+ can be used to turn off COSMO in a compound (multiple task)
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run. By default, once the COSMO solvation model has been defined it will
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be used in subsequent calculations. Add the keyword \verb+off+ if COSMO
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is not needed in subsequent calculations.
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\verb+Dielec+ is the value of the dielectric constant of the medium,
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with a default value of 78.4 (the dielectric constant for water).
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\verb+Radius+ is an array that specifies the radius of the spheres
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associated with each atom and that make up the molecule-shaped cavity.
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Default values are Van der Waals radii. Values are in units of angstroms.
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The codes uses the following Van der Waals radii by default:
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\begin{verbatim}
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data vdwr(103) /
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1 0.80,0.49,0.00,0.00,0.00,1.65,1.55,1.50,1.50,0.00,
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2 2.30,1.70,2.05,2.10,1.85,1.80,1.80,0.00,2.80,2.75,
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3 0.00,0.00,1.20,0.00,0.00,0.00,2.70,0.00,0.00,0.00,
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4 0.00,0.00,0.00,1.90,1.90,0.00,0.00,0.00,0.00,1.55,
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5 0.00,1.64,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,
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6 0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,
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7 0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,
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8 0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,
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9 0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,
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1 0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,1.65,
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2 0.00,0.00,0.00/
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\end{verbatim}
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with 0.0 values replaced by 1.80. Other radii can be used as well.
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See for examples:
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\begin{enumerate}
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\item E. V. Stefanovich and T. N. Truong, Chem.~Phys.~Lett. 244, 65 (1995).
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\item V. Barone, M. Cossi, and J. Tomasi, J.~Chem.~Phys. 107, 3210 (1997).
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\end{enumerate}
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\verb+Rsolv+ is a parameter used to define the solvent accessible
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surface. See the original reference of Klamt and Schuurmann for a
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description. The default value is 0.00 (in angstroms).
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\verb+Iscren+ is a flag to define the dielectric charge scaling option.
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``{\tt iscren 1}'' implies the original scaling from Klamt and Sch\"{u}\"{u}rmann,
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mainly ``$(\epsilon-1)/(\epsilon+1/2)$'', where $\epsilon$ is the dielectric constant.
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``{\tt iscren 0}'' implies the modified scaling suggested by Stefanovich and
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Truong, mainly ``$(\epsilon-1)/\epsilon$''. Default is to use the modified scaling.
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For high dielectric the difference between the scaling is not
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significant.
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The next three parameters define the tesselation of the unit sphere.
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The approach follows the original proposal by Klamt and Sch\"{u}\"{u}rmann.
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A very fine tesselation is generated from \verb+maxbem+ refining
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passes starting from either an octahedron or an icosahedron. The
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boundary elements created with the fine tesselation are condensed
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down to a coarser tesselation based on \verb+minbem+. The induced
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point charges from the polarization of the medium are assigned to
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the centers of the coarser tesselation. Default values are
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``{\tt minbem 2}'' and ``{\tt maxbem 3}''. The flag \verb+ificos+ serves to
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select the original tesselation, ``{\tt ificos 0}'' for an octahedron
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(default) and ``{\tt ificos 1}'' for an icoshedron. Starting from an icosahedron
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yields a somewhat finer tesselation that converges somewhat faster.
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Solvation energies are not really sensitive to this choice for
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sufficiently fine tesselations.
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The \verb+lineq+ parameter serves to select the numerical algorithm to solve
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the linear equations yielding the effective charges that represent
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the polarization of the medium. ``{\tt lineq 0}'' selects an iterative method
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(default), ``{\tt lineq 1}'' selects a dense matrix linear equation solver.
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For large molecules where the number of effective charges is large,
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the codes selects the iterative method.
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The following example is for a water molecule in `water', using
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the HF/6-31G** level of theory:
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\begin{verbatim}
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start
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echo
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title "h2o"
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geometry
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o .0000000000 .0000000000 -.0486020332
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h .7545655371 .0000000000 .5243010666
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h -.7545655371 .0000000000 .5243010666
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end
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basis segment cartesian
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o library 6-31g**
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h library 6-31g**
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end
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cosmo
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dielec 78.0
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radius 1.40
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1.16
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1.16
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rsolv 0.50
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lineq 0
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end
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task scf energy
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\end{verbatim}
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