Computational Chemistry

deviation from all three calculated results, and because it is among those dihedrals
which are said to be suspect or having a large or unknown error (designated X in
Harmony et al. – see reference in Table5.8). The error for the HOOH dihedral is
represents a clear failure of the HF/3–21G()level and is an example of a case
which provides an argument for using the 6–31G rather than the 3–21G(*)basis,

O

H H

0.948

0.969

(0.958)

C2v

HF / 6-31G*

MP2(fc) / 6 -31G*

Experiment

0.967

107.7

105.5

103.9

(104.5)

HF / 3-21G(*)

H C N

1.059

1.069

(1.065)

1.133

1.177

(1.153) C

iv

1.050 1.137

C2v

H

H

H

H

H H

1.528

1.526

(1.526))

H H

112.8

112.3

(112.4)

1.541

111.6

O O

H

H

C 2

1.473

1.397

1.469

0.970 (1.452)

0.949

0.976

(0.965)

99.5

102.1

98.6

(100.0)

H

H

H

H

H H

125.2

124.5

(124.3)

1.318

1.338

(1.318)

1.502

1.499

(1.501)

Cs

1.316

124.7

H C2v

H

H

H

H H

1.211 O

1.192

1.228

(1.222)

1.515

1.514

1.513

(1.507)

115.0

116.6

116.5

(117.2)

C3v

H

H

H

H

1.468

1.463

(1.459

1.466

1.188

1.187

1.220

(1.206)

O

H 115.7 115.6 H

(116.5)

C2v

1.083

1.092

1.104

(1.116)

1.207

1.184

1.221

(1.208)

114.9

D3d

H

H

H

H

HH

1.527

1.526

(1.531)

1.086

1.093

107.7 107.7 (1.096)

(107.8)

108.1

1.084

1.542

H

H

H

C3v

110.8 Cl

110.5

110.0

(110.0)

1.076

1.078

1.089

(1.096)

1.806

1.785

1.779

(1.781)

S

H H

1.326

1.340

(1.336)

94.4

93.3

(92.1)

94.2 C2v

1.327

H 105.1 Cs

102.6

(102.5)

0.951

0.979

(0.975)

1.670

1.717

O (1.690)

Cl

0.973 1.700

106.1

F

H

H

H

C3v

109.5

109.9

109.8

(110.6)

1.404

1.365

1.392

(1.383)

1.079

1.082

1.092

(1.100)

H

H

H

H

D2h

1.076

1.085

(1.085)

1.074

116.2

116.4

116.6

(117.8)

1.315

1.317

1.337

(1.339)

Cs

H

H

H

H

1.818

1.816

(1.819)

1.327

1.341

(1.336)

1.327

1.081

1.090

(1.091)

97.9

96.8

(96.5)

S

97.6

1.080

1.081

1.082

1.091

(1.091)

1.822

H

O

99.897.2 F

(96.8)

0.952

0.979

(0.966)

1.376

1.445

(1.442)

Cs

0.976 1.439

99.0 HHDih

1.051

1.057

1.066

(1.061)

1.188

1.186

1.218

(1.203)

S

Me O

Me

1.796

1.809

(1.799)

1.485

1.512

(1.485)

106.7

107.4

(106.7)

97.7

95.8

(96.6)

Cs

1.791 1.490

107.8

96.6

1.091

1.100

(1.099)

1.084

1.092

(1.099)

1.453

1.465

(1.471)

1.002

1.018

(1.010)

114.8

115.4

(113.9)

Cs

1.083

1.003

1.090

1.471

114.8

N

H

H

H

H H

O

H

H

H

H

1.0791.081 Cs

1.090

(1.094)

1.085

1.088

1.097

(1.094)

110.3

109.4

107.4

(108.0)

0.966

0.946

0.970

(0.963)

106.3

107.2

106.3

(107.2)

1.441

1.400

1.425

(1.421)

1.510

Fig. 5.23 A comparison of some HF/3–21G(), HF/6–31G and MP2(fc)/6–31G* geometries.
Calculations are by the author and experimental geometries are from ref. [ 1 g], Section 6.2. Note
that all CH bonds are ca. 1 A ̊, all other bonds range from ca. 1.2–1.8 A ̊, and all bond angles (except
for linear molecules) are ca. 90–120

284 5 Ab initio Calculations