Single Point Calculation

After running the 11-h2o-wfopt.inp with CPMD you have the ground state wavefunction stored in the file RESTART.1. Please rename it to RESTART so it won't get overwritten by the subsequent calculations.

A few comments on the differences of this input relative to the previous one.

This time we will use a gradient corrected functional (PBE) instead of the LDA, which can be confirmed by looking at the output:

 EXCHANGE CORRELATION FUNCTIONALS
    LDA EXCHANGE:                                            NONE
    LDA XC THROUGH PADE APPROXIMATION
    S.GOEDECKER, J.HUTTER, M.TETER PRB 54 1703 (1996)
    GRADIENT CORRECTED FUNCTIONAL
    DENSITY THRESHOLD:                                1.00000E-06
    EXCHANGE ENERGY
       [PBE: J.P. PERDEW ET AL. PRL 77, 3865 (1996)]
    CORRELATION ENERGY
       [PBE: J.P. PERDEW ET AL. PRL 77, 3865 (1996)]

Also note that in the &ATOMS section the LMAX for the Oxygen is set to P (instead of S for hydrogen) and that the keyword KLEINMAN-BYLANDER is added to choose the method for calculating the contributions of the nonlocal parts of the pseudopotential (see section 7.14.2). Again, this is reflected in the output:

 ****************************************************************
 *   ATOM       MASS   RAGGIO NLCC              PSEUDOPOTENTIAL *
 *      O    15.9994   1.2000  NO     KLEINMAN      S  NONLOCAL *
 *                                                  P     LOCAL *
 *      H     1.0080   1.2000  NO                   S     LOCAL *
 ****************************************************************

Changing it to GAUSS-HERMITE=30 (see section 7.14.1) gives us:

 ****************************************************************
 *   ATOM       MASS   RAGGIO NLCC              PSEUDOPOTENTIAL *
 *      O    15.9994   1.2000  NO GAUSS-HERMIT      S  NONLOCAL *
 *                                                  P     LOCAL *
 *                                GH INTEGRATION POINTS:     16 *
 *      H     1.0080   1.2000  NO                   S     LOCAL *
 ****************************************************************

Particularly for large systems the Kleinman-Bylander scheme is favored as the resulting number of non-local projectors is much smaller and thus much less work to do (i.e. the calculation runs faster). Also using a higher angular momentum as local potential is more efficient for the same reason. However, with the Kleinman-Bylander scheme, this can sometimes lead to unphysical ``ghost'' states (see section IV) and one has to choose a different local potential or even generate a new pseudopotential with different pseudization parameters to avoid them.

Even though we are requesting an isolated system calculation by setting SYMMETRY to ISOLATED SYSTEM, the calculation is, in fact, still done in a periodic cell. Since water has a dipole moment, we have to take care of the long range interactions and there are methods (see section 7.7 and POISSON SOLVER) implemented in CPMD to compensate for this effect. We go with the default, a HOCKNEY Poisson solver.