  High-level vibrational calculations have been used to investigate
anharmonicity in a wide variety of materials using density-functional-theory
(DFT) methods. We have developed a new and efficient approach for describing
strongly-anharmonic systems using a vibrational self-consistent-field (VSCF)
method. By far the most computationally expensive part of the calculations is
the mapping of an accurate Born-Oppenheimer (BO) energy surface within the
region of interest. Here we present an improved method which reduces the
computational cost of the mapping. In this approach we use data from a set of
energy calculations for different vibrational distortions of the materials and
the corresponding forces on the atoms. Results using both energies and forces
are presented for the test cases of the hydrogen molecule, solid hydrogen under
high pressure including mapping of two-dimensional subspaces of the BO surface,
and the bcc phases of the metals Li and Zr. The use of forces data speeds up
the anharmonic calculations by up to 40%.
