What is Slater’s rules?

Slater’s rules estimate the effective nuclear charge $Z^* = Z - \sigma$ felt by an electron. The electrons are grouped as (1s) (2s, 2p) (3s, 3p) (3d) (4s, 4p) (4d) (4f) (5s, 5p) …; the screening constant $\sigma$ of an electron is the sum of the contributions of the other electrons: 0.35 for each other electron of the same group (0.30 within 1s); for an s or p electron, 0.85 for each electron of the shell $n - 1$ and 1.00 for each electron of lower shells; for a d or f electron, 1.00 for every electron of the groups below its own. Groups above contribute nothing.