Vosko, Wilk, and Nusair form for exchange-correlation

The US National Institute of Standards and Technology (NIST) maintains a web site with density functional calculations of all atoms in the periodic table. So that we may compare our results directly with theirs, we must use exactly the same form for exchange-correlation. NIST employs the form suggested by Vosko, Wilk, and Nusair (VWN). Appendix C provides the routines exc() and excp() for this form. You should use these for your code, replacing all calls to excPZ() and excpPZ() with calls to exc(), and excp() respectively. (Hint: when placing exc() and excp() in ``physics.c'', you may need to put them before getVxc() and getDepsxc() to ensure the compiler has the proper prototypes.)