CASINO manual - Theory of Condensed Matter

The relativistic terms can be written as a sum of the mass-velocity term, Darwin terms and the
retardation term. The mass-velocity term ε 2 arises from the relativistic variation of mass with velocity,
and an estimator can be written as
ε 2 = − 1 ∑
8c 2 (∇ i · v i + |v i | 2 ) 2 . (276)
i
The spread of electronic charge is described by the electron–nucleus and electron–electron Darwin
terms
ε 3 + ε 4 = ∑ ∑ Z I π
2c 2 δ(r iI) + ∑ π
c 2 δ(r ij), (277)
I i
i