Combined quantum Monte Carlo - effective fragment molecular orbital method: fragmentation across covalent bonds.

The previously developed combined Quantum Monte Carlo-Effective Fragment Molecular Orbital (QMC-EFMO) method is extended to systems in which the fragmentation process cuts across covalent molecular bonds. The extended QMC-EFMO capability is demonstrated on a few model systems: the glycine tetramer, the diglycine reaction to form a dipeptide, silica-based rings, and polyalanine chains of increasing length. The agreement between full QMC and QMC-EFMO for the correlation energy is within 2 kcal mol-1 and for the correlation energy differences is within 1 kcal mol-1.

Functional derivatives of meta-generalized gradient approximation (meta-GGA) type exchange-correlation density functionals.

Meta-generalized gradient approximation (meta-GGA) exchange-correlation density functionals depend on the Kohn-Sham (KS) orbitals through the kinetic energy density. The KS orbitals in turn depend functionally on the electron density. However, the functional dependence of the KS orbitals is indirect, i.e., not given by an explicit expression, and the computation of analytic functional derivatives of meta-GGA functionals with respect to the density imposes a challenge. The practical solution used in many computer implementations of meta-GGA density functionals for ground-state calculations is abstracted and generalized to a class of density functionals that is broader than meta-GGAs and to any order of functional differentiation. Importantly, the TDDFT working equations for meta-GGA density functionals are presented here for the first time, together with the technical details of their computer implementation. The analysis presented here also uncovers the implicit assumptions in the practical solution to computing functional derivatives of meta-GGA density functionals. The connection between the approximation that is invoked in taking functional derivatives of density functionals, the non-uniqueness with respect to the KS orbitals, and the non-locality of the resultant potential is also discussed.

Systematic method to new phases of polymeric nitrogen under high pressure.

A systematic method to unravel a large class of single-bonded (SB) polymeric phases of nitrogen under high pressure is presented. The approach is based on the combinatorial generation of different Peierls-like distortions of a given reference structure that maintain the threefold connectivity of SB nitrogen, followed by first-principles calculations. Using an eight atom simple cubic reference structure, the approach not only recovers all four SB nitrogen phases reported to date, but eight new metastable structures (confirmed by phonon density of states calculations) are found. Basic properties of the structures are computed and the trends analyzed. Extensions to the method are straightforward and should lead to the discovery of more phases of polynitrogen.