2.5. Implementation
In this study, the pEDN (equations (2) and (3)) and pESP (equations (6) and (7)) were implemented in PAICS,39 which is our original FMO program package. In PAICS, EDN and ESP at the MP2 level of theory can be efficiently calculated by using RI approximation.24 In the current implementation, the grid data of the pEDN and pESP of proteins A and B are transferred to separate files, and the differential EDN is obtained by simply subtracting the pEDNs and used to define the PPI interface. To analyze the electrostatic complementarity of the PPI, the pESPs of proteins A and B are visualized at the PPI interface. Here, such a visualization of the grid data was performed with UCSF Chimera.40