From owner-chemistry@ccl.net Mon Dec 20 11:52:00 2010 From: "Theodore S. Dibble tsdibble*o*syr.edu" To: CCL Subject: CCL:G: Spin Contamination in Gaussian Message-Id: <-43422-101220115020-31680-FgBBO3pa2QpcLDeJyx47FA/a\server.ccl.net> X-Original-From: "Theodore S. Dibble" Date: Mon, 20 Dec 2010 11:50:18 -0500 Sent to CCL by: "Theodore S. Dibble" [tsdibble!^!syr.edu] The E(PMP2) results probably better, but "large" spin contamination can mean that the MP2 results are unreliable. What molecular system (including spin, charge, and symmetry) are you doing calcuations on, and are you doing MP2 optimizations or just single points? What is the spin before an after annihilation? CCSD calculations automatically eliminate the effect of the first spin contaminant. - Ted Dibble Theodore S. Dibble Chemistry Department SUNY-Environmental Science and Forestry 1 Forestry Drive Syracuse, NY 13210 (315) 470-6596 (315) 470-6856 (fax) http://www.esf.edu/chemistry/faculty/dibble.htm > "Decai Yu decaiyu23===yahoo.com" wrote: > > Sent to CCL by: Decai Yu [decaiyu23() yahoo.com] > Dear All, > > I am doing MP2 and CCSD calculations using Gaussian. > I found there is large spin contamination in the calculations. > But after annihilation, the spin contamination is smaller. > For the final energy, should I use E(PMP2) or EUMP2? > > For CCSD calculation, is the E(CORR) before or after annihilation? > > Anyhelp would be greatly appreciated! > > Decai Yu > >