07.04.2013 Views

Essentials of Computational Chemistry

Essentials of Computational Chemistry

Essentials of Computational Chemistry

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

9.3 SPECTROSCOPY OF NUCLEAR MOTION 339<br />

Table 9.2 Mean absolute errors in harmonic<br />

vibrational frequencies over a 32-molecule<br />

G2 subset (cm −1 ) a<br />

Level <strong>of</strong> theory Error<br />

MO theoretical methods<br />

HF/6-311G(3df,2p) 144<br />

MP2/6-31G(d,p) 99<br />

CCSD(T)/6-311G(3df,2p)<br />

LSDA functionals<br />

31<br />

SVWN/6-31G(d,p)<br />

GGA functionals<br />

75<br />

BLYP/6-311G(d,p) 59<br />

BPW91/6-311G(d,p) 69<br />

PWPW91/6-311G(d,p) 66<br />

mPWPW91/6-311G(d,p)<br />

Hybrid functionals<br />

66<br />

BH&HLYP/6-311G(d,p) 100<br />

B1LYP/6-311G(d,p) 33<br />

B1PW91/6-311G(d,p) 48<br />

mPW1PW91/6-311G(d,p) 39<br />

B3LYP/6-311G(d,p) 31<br />

B3PW91/6-311G(d,p) 45<br />

mPW3PW91/6-311G(d,p) 37<br />

a Test set includes 32 molecules containing only<br />

first-row atoms, see Johnson, Gill, and Pople<br />

(1993). Data from Adamo and Barone (1998).<br />

error remains. CCSD(T) and some <strong>of</strong> the hybrid levels <strong>of</strong> density functional theory show<br />

the highest accuracies. In general, the BLYP combination seems to be more accurate than<br />

BPW91, whether pure or hybrid in formulation, but PWPW91 is nearly as accurate as BLYP,<br />

again whether pure or hybrid in formulation.<br />

Of some interest in the error analysis is the degree to which the error is systematic.<br />

Although HF errors are large, they are very systematic. HF overemphasizes bonding, so<br />

all force constants are too large, and thus so are all frequencies. However, application <strong>of</strong><br />

a constant scaling factor to the HF frequencies improves their accuracy enormously (Pople<br />

et al. 1993). Scott and Radom studied this issue in detail for eight different levels <strong>of</strong> theory<br />

using a database <strong>of</strong> 122 molecules and 1066 fundamentals (i.e., measured, anharmonic vibrational<br />

frequencies) and a summary <strong>of</strong> their results, together with a few other recommended<br />

scaling factors, is provided in Table 9.3 (Scott and Radom 1996; see also, Wong 1996). Note<br />

that even though the scale factor required for the HF/6-31G(d) level <strong>of</strong> theory is substantial,<br />

reducing every frequency by more than 10%, the final accuracy is quite high – better<br />

than the considerably more expensive MP2. Note also that the pure DFT functional BLYP<br />

requires essentially no scaling, i.e., its errors are random about the experimental values,

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!