Theoretical investigation of isotope effects: The any-particle molecular orbital code

To study the hydrogen isotope effects in a series of diatomic molecules and water dimers we have created the any particle molecular orbital computer package (APMO). The current version of the APMO code is an implementation of the nuclear orbital and molecular orbital approaches (NMO) at the Hartree‐Fock level of theory. We have applied the APMO code to a variety of systems to elucidate the isotope effects on electronic wave functions, geometries and hydrogen bonds. We have studied the isotope effect on the dipole moments, electron densities and geometries of hydrogen molecule, lithium hydride and hydrogen fluoride and we have observed a reduction in the bond distance as the mass of the hydrogen isotopes is increased. This observation is in agreement with experimental data. We have also studied the primary and secondary isotope effects on the hydrogen bond of water dimers and we have observed that the hydrogen‐bond becomes weaker as the mass of the bonded hydrogen is increased. This trend has been observed by other authors. In contrast, the hydrogen bond becomes stronger when the mass of secondary hydrogens is increased. Our trends for secondary effects are in agreement with other theoretical and experimental studies. To our knowledge these are the first reported results on the secondary isotope effect on the hydrogen bond of water dimers using a NMO method. The applications presented in this paper demonstrate that the APMO code is highly suitable for the investigation of isotope effects in molecular systems containing a variety of quantum nuclei. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008