Fictitious electronic mass

In the following we will discuss with some more details the conditions that the fictitious electronic mass should satisfy. Let s first spend some short time illustrating the Car-Parrinello method on a system of 32 water molecules to simulate liquid water at ambient conditions. 

We have stated earlier that the fictitious electronic mass /x should be such that the electronic d3mamics is fast enough to follow adiabatically the ionic motion. We will try to give here some conditions for choosing /x [111, 178]. First, /X should be small enough so that the fictitious kinetic energy satisfies... 

The effective kinetic energy of the ions is modified by the fictitious electronic mass /i and this leads to uncertainties in the temperature of the system. This could be circumvented by defining the temperature as the configurational temperature [182] but this is too expensive in ab initio calculations and has not been tried so far. At a more empirical level, appropriate corrections to the atomic masses can be applied [183]. By satisfying the first condition given in the previous paragraph we can make the uncertainties for the temperature determination small. 

CPMD simulations were performed at constant volume and T = 84 K, which is the average temperature of different experimental studies of crystalline H502+C104 [103-105], In dynamic runs a fictitious electron mass of 600 au and a time step of 0,12 fs were employed. The BLYP equilibrium geometry was used as a starting point. The trajectory length was 4.5 ps. 

The fictitious electronic mass p in Eq. (12) can be chosen to be the same for all electronic degrees of freedom, alternatively it is also possible to assign... 

Although in CP-MD the nuclei are still treated as classical particles, the choice of time step can no longer be based solely on the highest nuclear frequency We also need to consider the fictitious dynamics of the electronic degrees of freedom. In fact, the optimum simulation time step is closely linked to the value of the fictitious electron mass ft as we wiU see in the following. 

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