From chemistry-request@server.ccl.net Sun Jan 20 20:39:44 2002 Received: from sirius.chem.vt.edu (IDENT:root@[128.173.181.42]) by server.ccl.net (8.11.6/8.11.0) with ESMTP id g0L1diI01403 for ; Sun, 20 Jan 2002 20:39:44 -0500 Received: from localhost (crawdad@localhost) by sirius.chem.vt.edu (8.11.0/8.11.0) with ESMTP id g0L1dQw20040; Sun, 20 Jan 2002 20:39:26 -0500 X-Authentication-Warning: sirius.chem.vt.edu: crawdad owned process doing -bs Date: Sun, 20 Jan 2002 20:39:26 -0500 (EST) From: "T. Daniel Crawford" X-Sender: To: Denny cc: Subject: Re: CCL:about size-consistent method In-Reply-To: <200201200212.g0K2CeI27210@server.ccl.net> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII On Sun, 20 Jan 2002, Denny wrote: > Dear all, > > Can someone comment something on size-consistent method? For example, I calculated potential energy curve for molecule AB > with B3LYP,CCSD(T) and QCISD(T) methods and aug-cc-pvtz even aug-cc-pvqz, the interaction including BSSE correction > E(AB,equilibrium position)-E(A,equilibrium)-E(B,equiblibrium) is -6.5eV, and E(AB,equilibrium)-E(A,single body)-E(B,single > body) is also about -6.5eV. However E(AB,nuclei distance R=1nm)-E(A,R=1nm)-E(B,R=1nm) is not 0eV but 0.8eV. Then the real > calculated dissociation energy is 6.5eV or (6.5+0.8)eV? I found CCSD(T) and QCISD(T) is also size-inconsistent though > someone say CCSD(T) is size-consistent. > > Any comments will be welcome!!! > > Best Regards, > Denny Chen > I'm not certain how to interpret all the calculations you describe above, but the basic definition of size-consistency involves non-interacting fragments. If you take two water molecules, for example, and place them an infinite distance apart, the energy you compute using a size-consistent method will be twice that for a single isolated water molecule. For this example, both QCISD(T) and CCSD(T) are size-consistent. One complication in the concept of size-consistency arises for closed-shell electronic states dissociating into open-shell fragments. If the non-interacting dimer involves an overall singlet state, for example, and the fragments are high-spin, then the single-determinant RHF method will not be size-consistent because two determinants are required for the dimer while only one is required for each high-spin fragment. I hope this helps. -TDC -- T. Daniel Crawford Department of Chemistry crawdad@vt.edu Virginia Tech www.chem.vt.edu/chem-dept/crawford Voice: 540-231-7760 FAX: 540-231-3255 -------------------- PGP Public Key at: http://www.chem.vt.edu/chem-dept/crawford/publickey.txt