Subject: about size-consistent method

From: Denny <[email-masked]>
Date: Sun, 20 Jan 2002 10:15:36 +0800

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

****************************
* Denny Chen		   *
* Center for Astrophysics  *
* Tsinghua University	   *
* Beijing, P.R.China       *
* 8610-62792126(phone)     *
* 8610-62792125(fax)       *
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Email:[email-masked]

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From: "T. Daniel Crawford" <[email-masked]>
Date: Sun, 20 Jan 2002 20:39:26 -0500 (EST)

> Any comments will be welcome!!!

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
[email-masked]                                    Virginia Tech
www.chem.vt.edu/chem-dept/crawford    Voice: 540-231-7760  FAX: 540-231-3255
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