Improve documentation for H2O force fields

data/forcefield_section/H2O/SPC.sec

@@ -3,6 +3,9 @@
     # https://doi.org/10.1021/j100308a038
     # Note: This force field requires a correpsonding &G3X3 constraint section
     #       in &MOTION / &CONSTRAINT
+    #       Alternatively, an unconstrained flexible variant of this force field can be defined
+    #       by setting the K parameters in the sections &BEND and &BOND to non-zero values.
+    #       Usage examples can be found in the folder cp2k/tests/Fist/regtest-water/.
     &FORCEFIELD
       &BEND
         ATOMS HW OW HW

data/forcefield_section/H2O/SPCE.sec

@@ -3,6 +3,9 @@
     # https://doi.org/10.1021/j100308a038
     # Note: This force field requires a correpsonding &G3X3 constraint section
     #       in &MOTION / &CONSTRAINT
+    #       Alternatively, an unconstrained flexible variant of this force field can be defined
+    #       by setting the K parameters in the sections &BEND and &BOND to non-zero values.
+    #       Usage examples can be found in the folder cp2k/tests/Fist/regtest-water/.
     &FORCEFIELD
       &BEND
         ATOMS HW OW HW

data/forcefield_section/H2O/TIP3P.sec

@@ -3,6 +3,10 @@
     # https://doi.org/10.1063/1.481505
     # Note: This force field requires a correpsonding &G3X3 constraint section
     #       in &MOTION / &CONSTRAINT
+    #       Alternatively, an unconstrained flexible variant of this force field can be defined
+    #       by setting the K parameters in the sections &BEND and &BOND to non-zero values, e.g.
+    #       see the file TIP3P_FLEXIBLE.sec.
+    #       Usage examples can be found in the folder cp2k/tests/Fist/regtest-water/.
     &FORCEFIELD
       &BEND
         ATOMS HW OW HW

data/forcefield_section/H2O/TIP3P_FLEXIBLE.sec

@@ -1,17 +1,19 @@
     # TIP3P flexible water
     # M. W. Mahoney and W. L. Jorgensen, J. Chem. Phys. 112, 8910 (2000)
     # https://doi.org/10.1063/1.481505
+    # https://docs.lammps.org/Howto_tip3p.html
+    # Usage examples can be found in the folder cp2k/tests/Fist/regtest-water/.
     &FORCEFIELD
       &BEND
         ATOMS HW OW HW
-        K [kcalmol*rad^-2] 110.0
-        KIND harmonic
+        K [kcalmol*rad^-2] 55.0
+        KIND charmm
         THETA0 [deg] 104.52
       &END BEND
       &BOND
         ATOMS OW HW
-        K [kcalmol*Angstrom^-2] 900.0
-        KIND harmonic
+        K [kcalmol*Angstrom^-2] 450.0
+        KIND charmm
         R0 [Angstrom] 0.9572
       &END BOND
       &CHARGE

tests/Fist/regtest-water/H2O-SPC-6x6x6.inp

@@ -173,6 +173,7 @@
       ELEMENT H
     &END KIND
     &TOPOLOGY
+      CONNECTIVITY generate
       MULTIPLE_UNIT_CELL ${na} ${nb} ${nc}
     &END TOPOLOGY
   &END SUBSYS

tests/Fist/regtest-water/H2O-SPCE-6x6x6.inp

@@ -173,6 +173,7 @@
       ELEMENT H
     &END KIND
     &TOPOLOGY
+      CONNECTIVITY generate
       MULTIPLE_UNIT_CELL ${na} ${nb} ${nc}
     &END TOPOLOGY
   &END SUBSYS

tests/Fist/regtest-water/H2O-TIP3P-6x6x6.inp

@@ -173,6 +173,7 @@
       ELEMENT H
     &END KIND
     &TOPOLOGY
+      CONNECTIVITY generate
       MULTIPLE_UNIT_CELL ${na} ${nb} ${nc}
     &END TOPOLOGY
   &END SUBSYS

tests/Fist/regtest-water/H2O-TIP3P_FLEXIBLE-6x6x6.inp

@@ -164,6 +164,7 @@
       ELEMENT H
     &END KIND
     &TOPOLOGY
+      CONNECTIVITY generate
       MULTIPLE_UNIT_CELL ${na} ${nb} ${nc}
     &END TOPOLOGY
   &END SUBSYS