Including nuclear quantum effects into highly correlated electronic structure calculations of weakly bound systems

Abstract

An interface between the APMO code and the electronic structure package MOLPRO is presented. The any particle molecular orbital APMO code [González et al., Int. J. Quantum Chem. 108, 1742 implements the model where electrons and light nuclei are treated simultaneously at Hartree-Fock or second-order Möller-Plesset levels of theory. The APMO-MOLPRO interface allows to include highlevel electronic correlation as implemented in the MOLPRO package and to describe nuclear quantum effects at Hartree-Fock level of theory with the APMO code. Different model systems illustrate the implementation: 4 He 2 dimer as a protype of a weakly bound van der Waals system; isotopomers of [He-H-He] + molecule as an example of a hydrogen bonded system; and molecular hydrogen to compare with very accurate non-Born-Oppenheimer calculations. The possible improvements and future developments are outlined.

On the physical interpretation of the nuclear molecular orbital energy

“…Previous studies have concluded that a systematic increase of the mass of a nucleus, treated quantum mechanically within a NMO approach, will gradually localize the nucleus bringing NMO derived properties closer to those calculated with BO based methods. 9,54,55 As a consequence, we expect that by gradually increasing the mass of nucleus Q, Q Z Q and V will approach V (r Q ). To test this conjecture, we increase systematically the mass of hydrogen nuclei in acetic acid.…”

The any particle molecular orbital approach: A short review of the theory and applications

“…The energy difference between the HF‐limit and the exact solution is known as the correlation energy . At the APMO full configuration interaction (APMO‐FCI) level, the wavefunction is constructed as a linear combination of all the possible excited determinants built from the reference HF wavefunction …”

“…Post‐HF MC methods have been proposed to recover interspecies correlation. Some examples of these methods are: configuration interaction (CI), Møller–Plesset (MP) many body perturbation theory, coupled‐cluster (CC), and propagator theory (PT) . In addition, MC Kohn–Sham density functional theory (DFT) has also been developed as an alternative to wavefunction methods to retrieve interspecies correlation …”

Decoupling electrons and nuclei without the Born-Oppenheimer approximation: The electron-nucleus mean-field configuration-interaction method

“…Then, one notices that nuclear cusps are smeared off in the Hamiltonian Eq. 7, and related basis set convergence issues may be removed 36 . Also, approximate excited states represented by products of k th i -excited functions, φ…”