Molecular wave functions and properties calculated using floating Gaussian orbitals

Abstract

The calculation of molecular electronic wave functions and properties using floating Gaussian orbitals (i.e., orbitals whose positions are optimized in space) is described. The wave function is optimized using a second-order convergent scheme (the trust-region method), and molecular properties up to second order are calculated analytically. The method is applied to a series of small molecules (HF, H2O, NH3, CH4, CO, H2CO, and C2H4) at the Hartree–Fock level using four different floating basis sets (double zeta, double zeta plus polarization, double zeta plus diffuse, and double zeta plus polarization and diffuse). Geometries are fully optimized, and dipole moments, static polarizabilities, harmonic frequencies, and double-harmonic infrared intensities are calculated at the optimized geometries. The results are compared with those obtained using the corresponding fixed basis sets, and also with the results from a large basis of near-Hartree–Fock quality [6–311++G(3df,3pd)]. Floating produces only minor changes in the electronic energy, but other properties are often significantly improved. In particular, properties involving external field variations (dipole moments, polarizabilities, and intensities) converge considerably faster to the Hartree–Fock limit when floating is allowed. Properties calculated using the floating double-zeta basis set augmented with one set of polarization functions and one set of diffuse orbitals are close to the Hartree–Fock limit.