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

An interface between the APMO code and the electronic structure package MOLPRO is presented. The any particle molecular orbital APMO code [González, Int. J. Quantum Chem. 108, 1742 (2008)]10.1002/qua.21584 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 high-level 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: ⁴He₂ 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.