Computational Chemistry

This survey suggests that:
For common organic molecules the Merck Molecular Force Field is nearly as
good as the ab initio MP2(fc)/6–31G* method for calculating geometries. Both

HH

Cs

C2v

O

H 104.0103.9 H

(104.5)

C2v

MMFF

Experiment

0.969

0.969

(0.958)

MP2(fc) / 6-31G*

HC N

1.065

1.069

(1.065)

1.160

1.177

(1.153) C

iv C

2v

H

H

H

H

H H

1.519

1.526

(1.526)

H H

111.7

112.3

(112.4)

1.454

1.469

(1.452)

O O

H

H

0.976

0.976

(0.965) 96.8

98.6

(100.0)

C 2

1.094

1.100

(1.099)

1.094

1.092

(1.099)

1.452

1.465

(1.471)

1.019

1.018

(1.010)

110.6

115.4

(113.9)

Cs

Cs

N

H

H

H

H H H

H

H

H

H H

124.2

124.5

(124.3)

1.339

1.338

(1.318)

1.493

1.499

(1.501)

109.0

106.3

(107.2)

O

H

H

H

H

0.972

0.970

(0.963)

1.093

1.097

(1.094)

1.093

1.090

(1.094)

107.1

107.4

(108.0)

Cs

1.416

1.425

(1.421)

H

H

H

H

H H

O

116.7

116.5

(117.2)

1.230

1.228

(1.222)

1.505

1.513

(1.507)

C3v

H

H

H

H

1.201

1.220

(1.206)

1.463

1.463

(1.459)

O

H H

1.225

1.221

(1.208)

1.102

1.104

(1.116)

115.5

115.6

(116.5)

C2v

D3d

H

H

H

H

HH

1.512

1.526

(1.531)

1.094

1.093

108.4 107.7 (1.096)

(107.8)

H

H

H

1.767

1.779

(1.781)

109.9

110.0

(110.0)

C3v

Cl

1.092

1.089

(1.096)

H 110.4 102.6 Cs

(102.5)

0.972

0.979

(0.975)

1.861

1.717

O (1.690)

Cl

S

H H

1.341

1.340

(1.336) 93.4

93.3

(92.1)

C 2 v

F

H

H

H

1.357

1.392

(1.383)

1.092

1.092

(1.100)

110.2

109.8

(110.6)

C3v

H

H

H

H

D2h

117.9

116.6

(117.8)

1.085

1.085

(1.085)

1.336

1.337

(1.339)

H

H

H

H

1.804

1.816

(1.819)

1.341

1.341

(1.336)

1.093

1.090

(1.091)

1.093

1.091

(1.091)

96.6

96.8

(96.5)

S

H

O

F

110.4

97.2

(96.8)

0.972

0.979

(0.966)

1.437

1.445

(1.442)

Cs

Dih

1.066

1.066

(1.061)

1.200

1.218

(1.203)

S

Me O

Me

1.809

1.809

(1.799)

1.500

1.512

(1.485)

107.5

107.4

(106.7)

95.8

95.8

(96.6)

Cs

(MMFF S / O single bond 1.806)

Fig. 3.13 A comparison of some MMFF, MP2(fc)/6–31G* and experimental geometries.
Calculations are by the author and experimental geometries are from ref. [30a]. 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 

68 3 Molecular Mechanics