Comparison of restricted, unrestricted, inverse, and dual kinetic balances for four-component relativistic calculations

摘要: Various kinetic balances for constructing appropriate basis sets in four-component relativistic calculations are examined great detail. These include the well-known restricted (RKB) and unrestricted (UKB) balances, less-known dual balance (DKB) as well unknown inverse (IKB). The RKB IKB complementary to each other: former is good positive-energy states, whereas latter negative-energy states. DKB combines of both even provides full variational safety. However, such an advantage largely offset by its complicated nature. UKB does not offer any particular advantages well. Overall, simplest ansatz. Although states a finite error O(c0), there no objection using them intermediates sum-over-states formulation perturbation theory, provided that magnetic also incorporated case properties. In particular, essential ingredient formulating two-component theories, while all others simply incompatible. As such, should be regarded cornerstone electronic structure calculations.