Ab initio molecular dynamics in adaptive coordinates.

We present a new formulation of ab initio molecular dynamics which exploits the efficiency of plane waves in adaptive curvilinear coordinates, and thus provides an accurate treatment of first-row elements. The method is used to perform a molecular dynamics simulation of the CO2 molecule, and allows to reproduce detailed features of its vibrational spectrum such as the splitting of the Raman σ g mode caused by Fermi resonance. This new approach opens the way to highly accurate ab initio simulations of organic compounds. Typeset using REVTEX 1 An increasing number of applications of ab initio molecular dynamics (MD) has given considerable insight into dynamical and statistical properties of a wide range of materials [1]. However, the application of this method to important fields such as organic chemistry, or to the study of oxide materials, has been hindered by difficulties in representing accurately first row elements using plane waves and pseudopotentials. Implementations of ab initio MD based on plane waves are intrinsically limited by the slow convergence of the Fourier expansions of wavefunctions and potentials. This limitation is particularly severe in the case of first row elements for which both wavefunctions and potentials are rapidly varying. Although some progress has been made recently to circumvent this problem by introducing modified pseudopotentials for first-row elements [2,3], a unified approach treating all atomic species on the same footing, and insuring the same accuracy and transferability of all potentials in various environments is still lacking. A method which relies on well established norm-conserving pseudopotentials [4], and does not necessitate special approximations for the representation of first row elements is essential e.g. for the study of materials under high pressure, or for MD simulations carried out at high temperatures. In such situations, interatomic distances can become substantially smaller than equilibrium distances, which requires a high transferability of pseudopotentials. Furthermore, it would be highly desirable to take advantage of the large body of information accumulated over the years about conventional norm-conserving pseudopotentials. In this paper, we present a new formulation of ab initio MD, which exploits the efficiency of plane waves in adaptive curvilinear coordinates, and therefore allows for an accurate treatment of all elements using conventional norm-conserving pseudopotentials. An application to the calculation of vibrational properties of the CO2 molecule yields results comparable to those obtained with the most elaborate quantum chemistry methods, but also allows to observe anharmonic effects on its vibrational spectrum. Our approach is based on the method of plane waves in adaptive coordinates which was recently proposed [5,6] to perform highly accurate electronic structure calcualtions, within the density functional, pseudopotential formalism. This method has been succesfully applied 2 to the determination of the equilibrium configuration of small molecules containing first-row elements [6] and to selected phases of SiO2 [7]. The basis set used in this method consists of plane waves in curvilinear coordinates