Małgorzata Jeziorska

Pair potential for helium from symmetry-adapted perturbation theory calculations and from supermolecular data

Symmetry-adapted perturbation theory (SAPT) was applied to the helium dimer for interatomic separations R from 3 to 12 bohrs. The first-order interaction energy and the bulk of the second-order contribution were obtained using Gaussian geminal basis sets and are converged to about 0.1 mK near the minimum and for larger R. The remaining second-order contributions available in the SAPT suite of codes were computed using very large orbital basis sets, up to septuple-zeta quality, augmented by diffuse and midbond functions. The accuracy reached at this level was better than 1 mK in the same region. All the remaining components of the interaction energy were computed using the full configuration interaction method in bases up to sextuple-zeta quality. The latter components, although contributing only 1% near the minimum, have the largest uncertainty of about 10 mK in this region. The total interaction energy at R=5.6 bohrs is -11.000+/-0.011 K. For R< or =6.5 bohrs, the supermolecular (SM) interaction energies computed by us recently turned out to be slightly more accurate. Therefore, we have combined the SM results for R< or =6.5 bohrs with the SAPT results from 7.0 to 12 bohrs to fit analytic functions for the potential and for its error bars. The potential fit uses the best available van der Waals constants C(6) through C(16), including C(11), C(13), and C(15), and is believed to be the best current representation of the Born-Oppenheimer (BO) potential for helium. Using these fits, we found that the BO potential for the helium dimer exhibits the well depth D(e)=11.006+/-0.004 K, the equilibrium distance R(e)=5.608+/-0.012 bohrs, and supports one bound state for (4)He(2) with the dissociation energy D(0)=1.73+/-0.04 mK, and the average interatomic separation R=45.6+/-0.5 A.

On the optimal choice of monomer geometry in calculations of intermolecular interaction energies: Rovibrational spectrum of Ar–HF from two- and three-dimensional potentials

Alternatives to using a full-dimensional interaction-potential energy surface and performing a complete dynamics on that surface have been examined for the Ar–HF van der Waals complex. We have employed a symmetry-adapted perturbation theory potential including the dependence on the H–F internuclear distance r. This potential was used to obtain a reference rovibrational spectrum of Ar–HF from the complete three-dimensional dynamics calculations. From the three-dimensional surface we have generated several two-dimensional potentials: the vibrationally averaged potential and the potentials obtained by fixing r at its equilibrium value re and at the vibrationally averaged distances 〈r−2〉−1/2, 〈r〉, 〈r2〉1/2, and 〈r3〉1/3. For all two-dimensional potentials obtained in this way the rovibrational spectra have been computed and compared with the reference spectrum. We have found that the potential obtained by setting r=〈r〉 performs much better than that corresponding to r=re. The spectrum closest to the reference o...

Portable parallel implementation of symmetry-adapted perturbation theory code

We describe a parallel implementation of symmetry-adapted perturbation theory of intermolecular interactions. Utilizing only sequential BLAS operations on distributed data structures and simplest message-passing calls for inter-process communication, the code is highly portable and runs efficiently on a variety of parallel systems, including the high-end shared- and distributed-memory machines, as well as on Linux clusters. A part of the code is a parallel implementation of the coupled cluster singles and doubles method, which has been shown to scale efficiently on as many as 64 processors. ∥This work is dedicated to Professor Andrzej J. Sadlej on the occasion of his 65th birthday.