Modified SSF partitioning function

[susilehtola]

Knuth et al described in Appendix C of Computer Physics Communications 190 (2015) 33–50 a modification to the Stratmann-Scuseria-Frisch partitioning function, which depends on the extent of the basis functions on the atoms. The modification is made to make the partitioning scale better in periodic systems, which would otherwise require very large integration volumes.