VASP Modifications
==========================

There are various modifications of VASP on the cluster, and others that are available but not installed on the cluster.

- Spin-orbit coupling: Installed as vasp_5.2_intel-parallel_so

- Selective relaxation: Modify the file ``constr_cell_relax.f90`` to change the stress tensor. For example, there's an executable with "isif27" that relaxes ions and volume, while keeping the shape constant; the stress tensor is forced to be a diagonal matrix with uniform diagonal elements. 

    Note: ISIF=7 relaxes volume and not using the code (in main.F)

    .. code-block:: fortran

        IF (DYN%ISIF==7) THEN
           DO I=1,3
           DO J=1,3
              D2SIF(J,I)=0
           ENDDO
           D2SIF(I,I)=PRESS*FACTSI
           ENDDO
        ENDIF

    The value of PRESS is calculated (in force.F) by 

    .. code-block:: fortran 

        PRESS=(TSIF(1,1)+TSIF(2,2)+TSIF(3,3))/3._q &
             &      -DYN%PSTRESS/(EVTOJ*1E22_q)*LATT_CUR%OMEGA

    Where TSIF and D2SIF are related by (in main.F)

    .. code-block:: fortran 

             D2SIF(I,K)=TSIF(I,K)*FACTSI

    These are different only if 

    Therefore, there is a difference between using the stress tensor based on the pressure vs. using it as a diagonal matrix with a=b=c; this difference is  ``-DYN%PSTRESS/(EVTOJ*1E22_q)*LATT_CUR%OMEGA``

    Iff PSTRESS=0, then this makes no difference. Beware.