28 research outputs found
Geometry Optimization of Crystals by the Quasi-Independent Curvilinear Coordinate Approximation
The quasi-independent curvilinear coordinate approximation (QUICCA) method
[K. N\'emeth and M. Challacombe, J. Chem. Phys. {\bf 121}, 2877, (2004)] is
extended to the optimization of crystal structures. We demonstrate that QUICCA
is valid under periodic boundary conditions, enabling simultaneous relaxation
of the lattice and atomic coordinates, as illustrated by tight optimization of
polyethylene, hexagonal boron-nitride, a (10,0) carbon-nanotube, hexagonal ice,
quartz and sulfur at the -point RPBE/STO-3G level of theory.Comment: Submitted to Journal of Chemical Physics on 7/7/0
Linear scaling computation of the Fock matrix. IX. Parallel computation of the Coulomb matrix
We present parallelization of a quantum-chemical tree-code [J. Chem. Phys.
{\bf 106}, 5526 (1997)] for linear scaling computation of the Coulomb matrix.
Equal time partition [J. Chem. Phys. {\bf 118}, 9128 (2003)] is used to load
balance computation of the Coulomb matrix. Equal time partition is a
measurement based algorithm for domain decomposition that exploits small
variation of the density between self-consistent-field cycles to achieve load
balance. Efficiency of the equal time partition is illustrated by several tests
involving both finite and periodic systems. It is found that equal time
partition is able to deliver 91 -- 98 % efficiency with 128 processors in the
most time consuming part of the Coulomb matrix calculation. The current
parallel quantum chemical tree code is able to deliver 63 -- 81% overall
efficiency on 128 processors with fine grained parallelism (less than two heavy
atoms per processor).Comment: 7 pages, 6 figure
Time-reversible Born-Oppenheimer molecular dynamics
We present a time-reversible Born-Oppenheimer molecular dynamics scheme,
based on self-consistent Hartree-Fock or density functional theory, where both
the nuclear and the electronic degrees of freedom are propagated in time. We
show how a time-reversible adiabatic propagation of the electronic degrees of
freedom is possible despite the non-linearity and incompleteness of the
self-consistent field procedure. Time-reversal symmetry excludes a systematic
long-term energy drift for a microcanonical ensemble and the number of
self-consistency cycles can be kept low (often only 2-4 cycles per nuclear time
step) thanks to a good initial guess given by the adiabatic propagation of the
electronic degrees of freedom. The time-reversible Born-Oppenheimer molecular
dynamics scheme therefore combines a low computational cost with a physically
correct time-reversible representation of the dynamics, which preserves a
detailed balance between propagation forwards and backwards in time.Comment: 4 pages, 4 figure
Density Matrix Perturbation Theory
An expansion method for perturbation of the zero temperature grand canonical
density matrix is introduced. The method achieves quadratically convergent
recursions that yield the response of the zero temperature density matrix upon
variation of the Hamiltonian. The technique allows treatment of embedded
quantum subsystems with a computational cost scaling linearly with the size of
the perturbed region, O(N_pert.), and as O(1) with the total system size. It
also allows direct computation of the density matrix response functions to any
order with linear scaling effort. Energy expressions to 4th order based on only
first and second order density matrix response are given.Comment: 4 pages, 2 figure