Ab Initio Calculation of Molecular Energies Including Parity Violating Interactions

A new approach, RHF-CIS, based on the perturbation of the ground state RHF wave function by the CIS excitations, has been implemented for evaluations of the parity violating interaction energy in molecules, E pv. The earlier approach, RHF-SDE, was based on the perturbation of the RHF ground states by the single-determinant “excitations” (SDE). The results obtained show the dramatic difference between E pv values in the RHF-CIS framework and those in the RHF-SDE framework: the E pv values of the RHF-CIS formalism are more than one order of magnitude greater compared to the RHF-SDE formalism as well as the corresponding tensor components. The maximum total value obtained for hydrogen peroxide as a function of torsional angle in the RHF-CIS framework is 3.661 × 10-19 E h (DZ** basis set) while the maximum E pv value for the RHF-SDE formalism is just 3.635 × 10-20 E h (TZ basis set). It is remarkable that both in the RHF-CIS and in the RHF-SDE approaches the diagonal tensor components of E pv strictly follow the geometry of a molecule and are always different from zero at chiral conformations. The zeros of the total E pv at chiral geometries are now found to be the results of the interplay between the diagonal tensor components. We have carried out an exhaustive analysis of the RHF-SDE formalism and have reproduced the related previous work. We studied how the RHF-SDE results vary when changing size and quality of basis sets. The RHF-SDE formalism generally fails to give adequate results and RHF-CIS must be considered as a first step for improvement. We will discuss the importance of the present results for possible measurements of the parity violating energy difference between enantiomers of chiral molecules.