Interfacing relativistic and nonrelativistic methods. IV. One- and two-electron scalar approximations

Two approximations for the inclusion of scalar relativistic effects into ot herwise nonrelativistic molecular self-consistent field calculations are de scribed, which continue the sequence of approximations from previous work. In the first, the two-electron contributions are confined to those where ea ch small component density is one-center. In the second, the two-electron c ontributions are approximated by using the bare Coulomb operator, an approa ch which parallels the Douglas-Kroll-Hess method. The results show that bot h approximations yield results close to the full two-electron scalar relati vistic method, within 0.001 Angstrom in bond lengths and 0.1 kcal/mol in di ssociation energies. The approximations also give results which are very si milar to those of the Douglas-Kroll-Hess method. The second approximation i s similar in cost to the Douglas-Kroll-Hess method and only contains simple operators whose integrals are easy to evaluate. (C) 2001 American Institut e of Physics.