Ab initio molecular dynamics techniques are widely employed to simulate equilibrium and non-equilibrium properties of molecular systems. [1] In particular, the so-called extended Lagrangian methods are a good compromise between accuracy and computational cost. In the Atom-centered Density Matrix Propagation (ADMP) approach, the one-electron density matrix is treated as a (classical) dynamical variable and propagated along with the nuclei. [2-5] Therefore, a fictitious electronic mass has to be introduced. This parameter has a fundamental role in governing the degree of adiabaticity between the (fictitious) electronic and (real) nuclear degrees of freedom.
A rational way to non-uniformly attribute masses to the density matrix elements in an orthonormal atomic basis is here proposed. To this end, a projection operator onto several molecular orbital subspaces is constructed. Both light and heavy atoms (treated as all-electron as well as with pseudopotentials) can be modelled with consistent accuracy. A detailed analysis of improvements and limits of the method performances is then presented. This approach can enhance adiabaticity between the electronic and nuclear degrees of freedom and allows for the employment of a wide range of time steps, without loss in accuracy. [6]

[1] Marx, D., Hutter, J. Ab initio molecular dynamics: basic theory and advanced methods. Cambridge University Press, 2009
[2] Schlegel, H.B. et al. J. Chem. Phys., 2001, 114, 9758-9763.
[3] Iyengar, S.S. et al. J. Chem. Phys., 2001, 115, 10291-10302.
[4] Iyengar, S.S. et al. Israel J. Chem., 2002, 42, 191-202.
[5] Rega, N. et al., J. Phys. Chem. B, 2004, 108, 4210-4220.
[6] Perrella, F., Petrone, A., Frisch, M.J., Rega, N. In preparation.

Dr. Michael Frisch

President, Gaussian, Inc.

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