An Examination Of The Effects Of Basis Set And Charge-Transfer In Hydrogen-Bonded Dimers With A Constrained Hartree-Fock Method

Constrained Hartree-Fock methods in which orbitals are constructed from strictly local nonorthogonal subsets of the molecule's atomic orbital basis have been known for over a decade. These methods have been principally used to generate localized molecular orbitals and interpret intramolecular interactions. In this paper, constrained Hartree-Fock results from basis sets ranging from minimal to extensive are presented for hydrogen-bonded dimers in which individual molecular orbitals are constructed from atomic orbitals belonging to individual monomers. These calculations eliminate both basis set superposition errors (BSSE) as well as charge transfer between monomers. This allows one to examine the effects of basis sets on the electrostatic and polarization components to the energy unmasked from BSSE. The charge transfer components are also isolated by comparing results to unconstrained calculations near the Hartree-Fock limit, where BSSE is vanishingly small. Finally, at moderate intermolecular separations when charge transfer becomes negligible, the constrained results are compared to both counterpoise-corrected and uncorrected unconstrained calculations.