How much do VSCF optimized curvilinear coordinates accelerate the convergence of state-specific VCI calculations?

The computational demands of vibrational configuration interaction (VCI) calculations are largely determined by the size of the correlation space. We show that this space can be substantially reduced in state-specific VCI calculations by using curvilinear coordinates optimized to minimize the vibrational self-consistent field ground-state energy. For fundamental, overtone, and combination bands of formaldehyde, fluoromethane, and trans-formic acid, optimized coordinates reduce the number of required configurations by factors of 3-7 compared to standard valence coordinates. The improvement is even greater for states in dense spectral regions with strong coupling between nearby states. For the C-H stretching excitations of trans-formic acid, optimized coordinates suppress resonant couplings and reduce the number of interacting states that must be tracked in state-specific VCI calculations, yielding configuration spaces being several tens of times smaller than with valence coordinates.