Q. Save core-memory by using Real*4 MO ERI (Electron Repulsion Integrals) in instead of the Real*8 one.

 

A. The SAC-CI program treats the MOERI in core memory. This means that we can perform larger systems if we can save the size of the MOERI. One way is to reduce the accuracy of the MOERI from Real*8 to Real*4. This reduction allows us to treat the active space by 1.2 times larger than the real*8 case. The result of the calculations does not show any meaningful changes as shown in the appendix of the SAC-CI guide.

  To generate the Real*4 executable (l923.exe), one can include USE_R4 option as an option for executing cpp. In alpha-osf1 case, include “-DUSE_R4” as a part of “ALPHADEF” in “alpha-osf1.make”.