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Ab initio calculation of both the lowest singlet and triplet electronic states of BN has been performed by the fixed-node Ornstein-Uhlenbeck diffusion quantum Monte Carlo method with the floating spherical Gaussian orbitals and spherical Gaussian geminals. The Monte Carlo calculation gives equilibrium bond lengths and equilibrium harmonic frequencies of 1.3317(7) Å and 1529(7) cm-1, respectively, for the lowest triplet state and 1.2751(7) Å and 1709(8) cm-1, respectively, for the lowest singlet state. Also, the Monte Carlo calculation reports an energy separation of 178(83) cm-1 between the two electronic states and recommends the ground state is the lowest triplet state. [ABSTRACT FROM AUTHOR]

This article accesses the performance of the Ornstein–Uhlenbeck diffusion quantum Monte Carlo with regard to the calculation of molecular geometries and harmonic frequencies of H[sub 2], LiH, HF, Li[sub 2], LiF, CO, N[sub 2], and F[sub 2] molecules. A comparison of the results for the eight first-row diatomic molecules from experiments, CCSD(T)/6-311G(3df,3pd) and CCSD(T)/cc-pV5Z levels of theory as well as our work is given. The results presented show that quantum Monte Carlo is becoming powerful tools for ab initio electronic structure calculations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This article describes an approach in determination of equilibrium geometries and harmonic frequencies of molecules by the Ornstein–Uhlenbeck diffusion quantum Monte Carlo method based on the floating spherical Gaussians. In conjunction with a projected and renormalized Hellmann–Feynman gradient and an electronic energy at variational Monte Carlo and diffusion quantum Monte Carlo, respectively, the quasi-Newton algorithm implemented with the Broyden–Fletcher–Goldfarb–Shanno updated Hessian was used to find the optimized molecular geometry. We applied this approach to N[sub 2] and H[sub 2]O molecules. The geometry and harmonic frequencies calculated were consistent with some sophisticated ab initio calculated values within reasonable statistical uncertainty.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Application of the Ornstein—Uhlenbeck diffusion quantum Monte Carlo method in combination with a trial wave function constructed from the floating spherical Gaussian orbitals and spherical Gaussian geminals to studies on the equilibrium structures and harmonic frequencies of ethane and ozone is presented. These Monte Carlo computed results are compared with those of experiments as well as the coupled cluster methods with the correlation consistent basis sets for the two molecules. For ozone, we also compare the Monte Carlo results with the results from multireference calculations. [ABSTRACT FROM AUTHOR]