The S02 card specifies the amplitude reduction factor $S_0^2$. If the entry for $S_0^2$ is less than 0.1, then the value of $S_0^2$ is estimated from atomic overlap integrals. Experimental values of $S_0^2$ are typically between 0.8 and 1.0.

Alternatively, you can specify the value of $S_0^2$ in the HOLE or EDGE card; however, the meaning of the parameters in the `feff.inp' file is more clear if you use the S02 card.

  * let FEFF calculate S02
  S02    0.0