Table 10. Relative Solvation Free Energy ΔΔGgas→aq(MM → QM) (in kcal/mol) with Different QM and QM-MM Electrostatic Hamiltoniansa.
| DFTB2 | SCC-DFTBPR | DFTB3/3OB | ||
|---|---|---|---|---|
| solute | MM ref. (Exp.)b | Coulomb | Klopman–Ohno (eq 5)c | Coulomb |
| H2O | –6.6 (−6.3) | +4.4 | +1.7 | |
| CH3COOH | –4.5 (−6.7) | –0.1 | –0.0 (+0.0) | –3.4 |
| CH3COO– | –81.9 (−77.6/–80.7163) | +3.1 | 13.1 (15.3) | –5.4 |
| CH3O– | –102.2 (−95.0) | +6.8 | 23.8 (24.9) | –3.9 |
| H3PO4 | –15.2 (−26.0) | –1.4 | –3.9 (−3.8) | –8.8 |
| H2PO4– | –80.8 (−76.0) | –1.7 | 10.1 (11.1) | –17.2 |
a
The MM here is the CHARMM22 force field. The water molecules are treated with TIP3P. Unless specified otherwise, the QM atoms use the standard CHARMM van der Waals parameters.
b
The “MM ref.” values are absolute solvation free energies computed following the standard protocol163,164 with periodic boundary conditions; no correction related to the gas/liquid interface has been included.165−167 Values in parentheses are experimental values from ref (168) for the first four solutes and from ref (169) for the phosphate species.
c
The values in parentheses use the CHARMM van der Waals parameters for the QM atoms; those without parentheses use the van der Waals parameters optimized in ref (73).