Adds one-electron operators oper1, oper2, ...with the corresponding factors fac1, fac2, ...to the one-electron hamiltonian. The available operators are given in section 6.13. An error will result if the added perturbation is not totally symmetric (symmetry 1).
FIELD+ adds to any existing field, otherwise any previous field is removed.
Note that FIELD does currently not modify core polarization potentials (CPP). If CPPs are present, only DIP and QUAD should be used.