Sent to CCL by: Adam Kubas [ad.kubas-#-gmail.com] Dear Yanbin, I think you should use ZPE(A+B)-ZPE(A)-ZPE(B). Moreover you have to remember that ZPE(A) and ZPE(B) have to be calculated on relaxed A and B geometries. Best regards, Adam -- Adam Kubas Karlsruher Institut für Technologie (KIT)