Predicting accurate vibrational frequencies for highly anharmonic systems

Improvements in the manner in which the potential energy surface (PES) is generated in the vibrational self-consistent field (VSCF) method have been implemented. The PES can now be computed over a flexible range of displacements and following normal mode displacement vectors expressed in internal rather than Cartesian coordinates, leading to higher accuracy of the calculated vibrational frequencies. The coarse-grained parallelization of the PES calculations, which is computationally by far the most expensive part of the VSCF method, enables the usage of higher levels of theory and larger basis sets. The new VSCF procedure is discussed and applied to three examples, H-3(+), HNO2, and HNO3, to illustrate its accuracy and applicability. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2987712]