Poisson-Boltzmann Solvation Model in Jaguar

With the standard Poisson-Boltzmann solver1 Keyword isolv = 2 in the gen section., you can compute solvation free energies and minimum-energy solvated structures or solvated transition states. The solvation free energy from a geometry optimization is computed as the difference between the energy of the optimized gas phase structure and the energy of the optimized solvated structure. Jaguar first calculates the usual gas phase wave function and from that the electrostatic potential, and fits that potential to a set of atomic charges, as described in Charges from Jaguar Electrostatic Potential Fitting.

These charges are passed to the Poisson-Boltzmann solver, which then determines the reaction field by numerical solution of the Poisson-Boltzmann equations and represents the solvent as a layer of charges at the molecular surface (which serves as a dielectric continuum boundary). These solvent point charges are returned to Jaguar’s SCF program, which performs another quantum mechanical wave function calculation, incorporating the solvent charges. This process is repeated until self-consistency is obtained. The cost is roughly twice that of a gas phase calculation. Jaguar performs a solvation calculation by setting the appropriate dielectric constant2 Keyword epsout in the gen section. and probe radius for the solvent.3 Keyword radprb in the gen section. The dielectric constant [104] and probe radius values set by Jaguar for various solvents are shown in Table 1. A full list of solvents is given in Solvation Keywords in the Jaguar Input File.

Solvation free energies can be computed with HF, DFT, LMP2, CIS, and TDDFT wave functions. For local LMP2 solvation free energy calculations, we recommend using heteroatom pairs for the most efficient results, particularly since solvation free energy calculations often use radii optimized for calculations that use heteroatom pairs. See The PBF Radii File for Jaguar Calculations for more details; see Local MP2 Settings for information on setting LMP2 pairs.

In addition to calculating solvated free energies, you can also calculate properties analytically for solvated species. The CPHF equations are solved in the reaction field to produced solvated perturbed wave functions, which are then used to calculate the properties.

To use a solvent that is not on this option menu, you can define it by setting keywords for the probe radius (radprb) and the dielectric constant (epsin) in the Keywords text box. The probe radius can be calculated using the expression given here, in the Solvation Keywords in the Jaguar Input File topic.

If you compute the solvation free energy of a minimum-energy or transition-state structure optimized in solution, your calculation should compare the energy of the optimized solvated structure to the energy of the optimized gas-phase structure. Therefore, by default, geometry optimizations in solution are performed only after an optimized gas-phase structure is computed.4 Keyword nogas = 0 in the gen section. However, if you want only an optimized structure in solution and do not care about the computed solvation free energy, you can skip the gas phase geometry optimization by selecting Input structure5 Keyword nogas = 2 in the gen section. or Value6 Keyword nogas = 1 in the gen section. under Gas phase reference energy. These options allow you to use a reference energy other than that of the optimized gas phase structure.