We propose a new simulation method, which combines a cage model and a density of states partitioning technique, to compute the free energy of an arbitrary solid. The excess free energy is separated into two contributions, noninteracting and interacting. The excess free energy of the noninteracting solid is computed by partitioning its geometrical configuration space with respect to the ideal gas. This quantity depends on the lattice type and the number of molecules. The excess free energy of the interacting solid, with respect to the noninteracting solid, is calculated using density of states partitioning and a cage model. The cage model is better than the cell model in that it has a smaller configuration space and better represents the equilibrium distribution of solid configurations. Since the partition function (and hence free energy) is obtained from the density of states, which is independent of the temperature, equilibrium thermodynamic properties at any condition can be obtained by varying the density. We illustrate our method in the context of the free energy of dry ice.

Iterated stockholder atoms are produced by dividing molecular electron densities into sums of overlapping, near-spherical atomic densities. It is shown that there exists a good correlation between the overlap of the densities of two atoms and the order of the covalent bond between the atoms (as given by simple valence rules). Furthermore, iterated stockholder atoms minimise a functional of the charge density, and this functional can be expressed as a sum of atomic contributions, which are related to the deviation of the atomic densities from spherical symmetry. Since iterated stockholder atoms can be obtained uniquely from the electron density, this work gives an orbital-free method for predicting bond orders and atomic anisotropies from experimental or theoretical charge density data.

The intermolecular potentials for the Ne–HF complex have been calculated using MP2 and SIMPER-P methods. A detailed analysis of the intermolecular potential is carried out using the perturbation theory. The energies of Van der Waals rovibrational bound states are calculated from the potential energy surfaces, and compared with previously published high-resolution spectra. SIMPER-P method is shown to produce results competitive to high-level CCSD(T) method and to be in good agreement with experimental results.SIMPER-P method for van der Waals complexes is applied to the Ne–HF complex. The method is shown to be competitive to CCSD(T) method in accuracy, while being computationally efficient.

A new, simple, computational technique allows molecular electron densities to be divided into atoms which have intuitively correct shapes and charges.

We use the distributed multipole analysis method to analyse the charge density of the hydrogen fluoride molecule, including variation of the HF bond length, and taking into consideration the level of theory, the basis set, and the number of sites used. We also examine the effects of truncating the dimer electrostatic interactions at charge–charge, quadrupole–quadrupole and all interactions up to r−5 dependence in the intermolecular site–site separations. We assess the accuracy of these approximations using both the calculated multipoles and multipoles described by polynomial functions of the bond stretching coordinate. We consider two ranges of the HF bond length, one of which should be suitable for calculations of structures and energetics of hydrogen fluoride clusters, and the other for dynamical calculations on the same systems.

Iterated stockholder atoms are produced by dividing molecular electron densities into sums of overlapping, near-spherical atomic densities. It is shown that there exists a good correlation between the overlap of the densities of two atoms and the order of the covalent bond between the atoms (as given by simple valence rules). Furthermore, iterated stockholder atoms minimise a functional of the charge density, and this functional can be expressed as a sum of atomic contributions, which are related to the deviation of the atomic densities from spherical symmetry. Since iterated stockholder atoms can be obtained uniquely from the electron density, this work gives an orbital-free method for predicting bond orders and atomic anisotropies from experimental or theoretical charge density data.

A new, simple, computational technique allows molecular electron densities to be divided into atoms which have intuitively correct shapes and charges.