CRYSTALS program

MJ Field, M Karplus. CRYSTAL Program for Crystal Calculations in CHARMM. Cambridge, MA Harvard University, 1992. 

The first computational consideration is that of obtaining the solutions of the unperturbed problem, Eq. (15), and the approach taken in the present study is to utilize the Crystal program [1] as it has been successfully used for studies in molecular crystals [10-12,15], A given crystalline orbital, (k,r), such as that required for the matrix elements necessary given by the integral in Eq. (16), is expressed as a linear combination of Bloch functions, a ,(k) and atomic orbitals, (k,r) [1]... 

Over the past twenty years, the available software for Hartree-Fock (HF) and for density-functional calculations on crystals has undergone very significant progress in terms of effectiveness and reliability. For example, the crystal program package is today applied routinely to a wide range of problems. 

This work was supported by Science Foundation Ireland under grant number 02/IN. 1/147. NG and ALR thank the DFG (Photonic Crystals Program) for financial support. 

Table 1 Optimised energies (in kcal/mol relative to the RHF calculation) of the models for the dimerisation. = SYMM is indicated when the optimisation led to the symmetric dimer. The UHF singlet is calculated with opposite spins on the 2 atoms of the SigHn dimer (it is an asymmetric component of the covalent singlet state calculated with the crystal program and the geometry of the triplet).

It is now possible to compare the molecular scheme with the main steps of the CRYSTAL program ... 

Figure A2.2 Total elapsed time for HF total energy calculations of MgO cubic supercells with 8, 16, 32, 64, 128, 256, and 512 ions. The CRYSTAL program was compiled using the Intel Fortran Compiler IFC7.0 with the —02 —tp p7 options. Calculations were carried out with a Pentium Xeon 2.4-GHz single-processor computer, with 2-GB RAM, 512-KB cache, wide SCSI disks.

CRYSTALS program package for crystal structure determination, http //www.xtl.ox.ac.uk. 

Orlando R, Dovesi R, Ugliengo P, (1999) A quantum mechanical periodic ab initio approach to materials science the CRYSTAL program. Int J Inorg Mater 1 147-155 Oum KW, Lakin MJ, DeHaan DO, Brauers T, Finlayson-Pitts BJ (1998) Formation of molecular chlorine from the photolysis of ozone and aqueous sea-salt particles. Science 279 74-77 Paizs B, Suhd S (1997) Extension of SCF and DFT versiorrs of chemical Hamiltonian approach to N interacting subsystems and an algorithm for their eflficierrt implementatioa J Comput Chem 18 694-701... 

The local MP2 electron-correlation method for nonconducting crystals [109] is an extension to crystalline solids of the local correlation MP2 method for molecules (see Sect. 5.1.5), starting from a local representation of the occupied and virtual HF subspaces. The localized HF crystalline orbitals of the occupied states are provided in the LCAO approximation by the CRYSTAL program [23] and based on a Boys localization criterion. The localization technique was considered in Sect. 3.3.3. The label im of the occupied localized Wannier functions (LWF) Wim = Wj(r — Rm) includes the type of LWF and translation vector Rm, indicating the primitive unit cell, in which the LWF is centered (m = 0 for the reference cell). The index i runs from 1 to A i, the number of filled electron bands used for the localization procedure the correlation calculation is restricted usually to valence bands LWFs. The latter are expressed as a linear combination of the Gaussian-type atomic orbitals (AOs) Xfiif Rn) = Xfin numbered by index = 1,..., M M is the number of AOs in the reference cell) and the cell n translation vector... 

HF space is expressed in terms of the density matrix in an AO representation, P, which is calculated very accurately by the CRYSTAL program via BriUouin-zone integration over all canonical occupied crystaUine orbitals / ... 

This implementation requires simple modifications in the property part of the CRYSTAL program [23]. A Lowdin population analysis is introduced for self-consistent DM and the bond-order sums are calculated for atoms of the crystal. The lattice summation in (6.76) is made over the same part of the lattice that has been used in the integrals calculation for the self-consistent procedure (the lattice summation field is defined by the most severe tolerance used in the two-electron exchange integrals calculation). ... 

In the past, it would take months or years to determine the structure of a protein molecule now it takes a day or even a few hours. Since the wavelength can be changed easily and the intensity of an x-ray is much brighter, more data can be collected. By analyzing the diffraction pattern using software, it becomes much simpler to make a map of the electron density in the crystal. Programs are available or may be devised to have the shape and the structure displayed on a computer screen as a mass of squiggly lines, ribbons, and little balls. 

We wish to express our thanks to the N.S.E.R.C. (Canada) and S.E.R.C. (U.K.) for financial support and Dr. David J. Watkin, Chemical Crystellography Laboratory, University of Oxford, for carrying out the refinement calculation with the CRYSTALS program. 

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