Accuracy of Prediction of Vibrational Frequencies (Home)

Vibrational frequencies are used in the calculation of entropy and heat capacity, and in starting time-dependent simulations involving the Dynamic Reaction Coordinate.  PM6 does a poor job of estimating frequencies, as is shown in the graph and table below.  All reference data is taken from the Handbook of Chemistry and Physics.

 

Statistics for the set of 115 vibrations species

Average unsigned error:

   114.2cm-1

Average signed error:

   -62.3cm-1

Average unsigned error: 

  14%

Average signed error: 

  -6%

 

Each of the PM6 results given below was generated by doing two calculations.  First, the geometry was optimized, then a FORCE calculation was done.  These calculations were done in a single run, with the data set having the form:

<keywords: none used>
<Name of species>
<Blank>
<Z-matrix: For examples, see: ref-data>
<Blank line>
 OLDGEO FORCE
<Blank line>
<Blank line>

 

 

Vibrational Frequencies for Individual Species used in Preparing this Graph
(All values are in cm-1)

Molecule

Ref.Vib.

PM6 Vib.

Diff.

% Diff.

Carbon dioxide

667

539

-128

-19.2

1333

1377

44

3.3

2349

2320

-29

-1.2

Carbon disulfide

397

361

-36

-9.1

658

650

-8

-1.2

1535

1364

-171

-11.1

Water

1595

1336

-259

-16.2

3657

2527

-1130

-30.9

3756

2613

-1143

-30.4

Heavy water

1178

992

-186

-15.8

2671

1854

-817

-30.6

2677

1856

-821

-30.7

F2O

461

469

8

1.7

831

1096

265

31.9

928

1119

191

20.6

Cl2O

296

322

26

8.8

639

810

171

26.8

686

869

183

26.7

H2S

1183

1057

-126

-10.7

2615

2685

70

2.7

2626

2695

69

2.6

HCN

712

975

-263

36.9

2097

2246

-149

7.1

3311

2818

493

-14.9

Ammonia

950

1133

-183

19.3

1627

1644

-17

1.0

3337

2837

500

-15.0

3444

2898

546

-15.9

Phosphine

992

997

-5

0.5

1118

1133

-15

1.3

2323

2756

-433

18.6

2328

2761

-433

18.6

Arsine

906

974

-68

7.5

1003

1037

-34

3.4

2116

2320

-204

9.6

2123

2344

-221

10.4

PF3

344

239

105

-50.9

487

318

169

-63.0

860

861

-1

0.1

892

865

27

-3.0

Methane

T

1306

1254

52

-4.0

E

1524

1262

262

-17.2

A

2917

2825

92

-3.2

T

3019

2719

300

-9.9

CF4

E

435

356

79

-18.2

T

632

497

135

-21.4

A

909

988

-79

8.7

T

1281

1391

-110

8.6

CCl4

E

217

212

5

-2.3

T

314

316

-2

0.6

A

459

553

-94

20.5

T

776

825

-49

6.3

CBr4

E

122

110

12

-9.8

T

182

173

9

-4.9

A

267

311

-44

16.5

T

672

717

-45

6.7

CI4

E

90

61

29

-32.2

T

125

105

20

-16

A

178

200

-22

12.4

T

555

635

-80

14.4

SiH4

T

914

895

19

-2.1

E

975

899

76

-7.8

A

2187

2072

115

-5.3

T

2191

2113

78

-3.6

SiF4

E

268

214

54

-20.1

T

389

244

145

-37.3

A

800

574

226

-28.3

T

1032

784

248

-24.0

SiCl4

E

150

111

39

-26.0

T

221

169

52

-23.5

A

424

371

53

-12.5

T

621

586

35

-5.6

GeH4

T

819

745

74

-9.0

E

931

856

75

-8.1

A

2106

2382

-276

13.1

T

2114

2260

-146

6.9

GeCl4

E

134

96

38

-28.4

T

172

137

35

-20.3

A

396

368

28

-7.1

T

453

405

48

-10.6

SnCl4

E

104

88

16

-15.4

T

134

115

19

-14.2

A

389

346

43

-11.1

T

403

375

28

-6.9

TiCl4

E

114

93

21

-18.4

T

136

121

15

-11

A

389

307

82

-21.1

T

498

389

109

-21.9

ZrCl4

E

98

79

19

-19.4

T

113

104

9

-8.0

A

377

302

75

-19.9

T

418

356

62

-14.8

HfCl4

E

102

106

-4

3.9

T

112

108

4

-3.6

A

382

379

3

-0.8

T

390

356

34

-8.7

RuO4

E

322

270

61

-18.9

T

336

261

66

-19.6

A

885

811

117

-13.2

T

921

768

110

-11.9

OsO4

T

329

308

73

-22.2

E

333

256

25

-7.5

T

960

861

99

-10.3

A

965

865

100

-10.4

Acetylene

612

796

-184

30.1

730

941

-211

28.9

1974

2235

-261

13.2

3289

2832

457

-13.9

3374

2928

446

-13.2

Formaldehyde

1167

1101

66

-5.7

1249

1156

93

-7.4

1500

1351

149

-9.9

1746

1794

-48

2.7

2783

2615

168

-6.0

2843

2666

177

-6.2