Ab-Initio Molecular Dynamics of Organic Compounds on a Massively Parallel Computer
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FRANCOIS GYGI Institut Romand de Recherche Num6rique en Physique des Mat6riaux (IRRMA) CH-1015 Lausanne, Switzerland
ABSTRACT We present results of ab-initio electronic structure calculations and molecular dynamics simulations of organic molecules carried out using adaptive curvilinear coordinates, within the local density approximation of density functional theory. This approach allows for an accurate treatment of first-row elements, which makes it particularly suitable for investigations of organic compounds. A recent formulation of this method relies on a real-space approach which combines the advantages of finite-difference methods with the accuracy of adaptive coordinates, and is well suited for implementation on massively parallel computers. We used molecular dynamics simulations to obtain the fully relaxed structures of nitrosyl fluoride (FNO), and of the aromatic heterocycles furan and pyrrole. The equilibrium geometries obtained show excellent agreement with experimental data. The harmonic vibrational frequencies of furan and pyrrole were calculated by diagonalization of their dynamical matrix 1 and are found to agree with experimental data within an rms error of 25 cm- and 28 cm-1 for furan and pyrrole respectively. This accuracy is comparable to that attained for smaller organic molecules using elaborate quantum chemistry methods. INTRODUCTION Ab-initio molecular dynamics simulations of organic compounds represent a challenge for most electronic structure methods currently in use. This difficulty is due to the presence in organic molecules of first-row elements whose wavefunctions are rapidly varying in space and therefore require a highly accurate numerical representation. Recent developments in abinitio electronic structure have focused on the need to develop efficient molecular dynamics methods capable of treating all elements of the periodic table on an equal footing. Among them, the adaptive coordinate method [1] was recently proposed to improve the efficiency of the plane wave approach, and has been used in MD simulations including first-row elements [2]. The applicability of this method to the calculation of structural properties of solids and to the computation of band structures has also been demonstrated [3]. Recently, the adaptive coordinate method was reformulated using a real-space approach [4], which retains the simplicity of the recently introduced finite-difference formulation [5], while increasing is accuracy. The implementation of this method on a parallel computer is particularly straightforward since the real-space grid can be partitioned and distributed among processors. Most operations are then carried out locally on each processor, and interprocessor communications are kept to a minimum. In particular, no Fourier transforms are used in the calculation. The method is implemented on a Cray-T3D massively parallel computer. This approach is well suited to the calculation of the electronic structure of molecules containing first-row 131 Mat. Res. Soc. Svmp. Proc. Vol. 408 01996 Materia
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