Re: CCL:about size-consistent method

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 "-at-" 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