Concise Physical Chemistry

c20 JWBS043-Rogers September 13, 2010 11:29 Printer Name: Yet to Come

338 QUANTUM MOLECULAR MODELING

HF/6-31G(d) FOpt HF geometry

HF/6-31G(d) Freq ZPE and vib freq

MP2(full)/6-31G(d) Opt MP2(full) geo

QCISD(T)/6-31G(d) QCI energy

MP2/GTMP2Large GTMP2Large: energy

Scheme 20.1 A computational chemical script. The G3MP2Large set also goes under the

names G3MP2large and GTMP2Large. (If one input notation gives you an error message, try

one of the others.)

The sum of all these five energy terms with a G3 basis set at the MP2 post-

Hartree–Fock level isE 0 [G3(MP2)]:

E 0 [G3(MP2)]=E(QCISD(T)/6–31G(d))+ EMP2+ E(SO)

+E(HLC)+E(ZPE)

20.17 COMBINED ORSCRIPTEDMETHODS

A quantum mechanicalscriptis a list of procedures that are carried out sequentially

and automatically once the program has been started. Combined or scripted programs

like the GAUSSIAN©C family and the CBS©Cgroup by Petersson (1998) are very

popular. A sample script is shown as Scheme 20.1.

Using the results from the scripted sequence, an extrapolation can be carried out

as shown in Fig. 20.8. The desired energyE[(QCISD(T)/G3MP2Large] cannot be

obtained by direct calculation, but the other three energies can. With these three

energies, extrapolation to the desired energy is carried out as diagramed in the fig-

ure. The line fromE[MP2/6-31G(d)] toE[MP2/GTMP2Large] represents lowering

of the MP2/6-31G(d) energy when the basis set is expanded to MP2/GTMP2Large.

The line fromE[MP2/6-31G(d)] toE[QCISD(T)/6-31G(d)] is the 6-31G(d) energy

lowering when the QCISD(T) correlation is taken into account.

All of the calculations above are carried out within the G3(MP2) script. The

output portion of the G3(MP2) printout ofH^298 for methanol shown in File 20.11

contains the messageG3MP2 Enthalpy = -115.547933.This value inserted

into Fig 20.7 asH^298 gives (^) fH^298 (methanol)=− 199 .4kJmol−^1 as compared to
the experimental value of –201.5±0.3 kJ mol−^1. The difference is 1%. We have
reached our accuracy goal for methanol and, by implication, for any molecule we
choose, subject only to limitations on computer power.