return to home page Computational Chemistry Comparison and Benchmark DataBase Release 22 (May 2022) Standard Reference Database 101 National Institute of Standards and Technology
You are here: Calculated > Geometry > Calculated geometry > One type of bond

Comparison of levels of theory for O-O


Click on an entry for a histogram of the values.
The subscript refers to how many bonds of that type have been calculated.
average calculated bond lengths (Å)
Methods with predefined basis sets
semi-empirical AM1 1.2420
32
PM3 1.3735
32
PM6 1.2820
45
composite G2 1.3437
37
G3 1.3439
37
G3B3 1.3818
41
G3MP2 1.2697
8
G4 1.3630
45
CBS-Q 1.3316
38

average calculated bond lengths (Å)
Methods with standard basis sets
STO-3G 3-21G 3-21G* 6-31G 6-31G* 6-31G** 6-31+G** 6-311G* 6-311G** 6-31G(2df,p) 6-311+G(3df,2p) 6-311+G(3df,2pd) TZVP cc-pVDZ cc-pVTZ cc-pVQZ aug-cc-pVDZ aug-cc-pVTZ aug-cc-pVQZ cc-pV(T+d)Z cc-pCVDZ cc-pCVTZ cc-pCVQZ aug-cc-pCVTZ Sadlej_pVTZ daug-cc-pVDZ daug-cc-pVTZ
hartree fock HF 1.3707
42
1.4152
42
1.9285
43
1.3871
42
1.3227
49
1.3295
43
1.3187
39
1.3163
43
1.3203
42
1.3198
43
1.3755
20
1.1579
1
1.3130
52
1.3218
43
1.3194
43
1.3070
36
1.3226
41
1.3181
42
1.2911
28
1.2737
1
1.1590
1
1.2298
2
1.1578
1
1.3111
3
1.1640
1
1.3939
12
1.3034
57
ROHF 1.2581
5
1.3284
18
1.3293
18
1.2760
16
1.2365
17
1.2362
17
1.2358
17
1.2248
17
1.2223
16
1.1777
5
1.2062
4
1.1579
1
1.2253
8
1.2279
17
1.2268
17
1.1995
13
1.2217
15
1.2194
15
1.2039
12
  1.1541
1
1.2283
2
1.1509
1
1.2317
2
  1.2258
5
1.2233
5
density functional LSDA 1.3854
34
1.4265
33
1.4267
34
1.3850
32
1.3239
33
1.3226
33
1.3220
33
1.3168
33
1.3401
34
1.3153
32
1.2014
1
1.2014
1
1.3182
19
1.3170
33
1.3159
33
1.2494
5
1.3162
32
1.3079
27
1.2568
4
  1.2078
1
1.2572
2
1.2026
1
1.3315
3
  1.2645
2
1.2609
2
BLYP 1.4480
43
1.4807
42
1.4838
43
1.4575
43
1.3775
53
1.3968
43
1.3995
42
1.3957
42
1.3940
41
1.3881
41
1.5206
12
1.2283
1
1.3947
27
1.3951
44
1.3906
43
1.2295
1
1.4056
21
1.4038
18
1.2288
1
  1.2344
1
1.2943
2
1.2293
1
1.3874
3
  1.4917
12
1.4894
12
B1B95 1.4096
42
1.4515
39
1.4459
42
1.4099
41
1.3594
41
1.3558
42
1.3531
42
1.3482
42
1.3486
42
1.3472
38
1.4559
12
1.1916
1
1.3457
27
1.3504
42
1.3485
42
1.2437
5
1.3427
37
1.3427
36
1.2413
5
  1.1958
1
1.2524
2
1.1921
1
1.3341
3
  1.4013
8
1.3117
7
B3LYP 1.4227
43
1.4523
44
1.4546
44
1.4223
43
1.3678
44
1.3677
44
1.3635
43
1.3599
44
1.3599
41
1.3597
46
1.4208
17
1.2033
1
1.3567
52
1.3680
44
1.3649
43
1.3431
30
1.3524
40
1.3597
43
1.3359
27
1.1950
1
1.2082
1
1.2669
2
1.2038
1
1.3544
3
1.2123
1
1.4531
12
1.4506
12
B3LYPultrafine 1.2853
1
1.4712
18
1.3005
1
1.2548
1
1.3646
40
1.3825
18
1.3600
21
1.3755
18
1.2056
1
1.4853
12
1.4765
12
1.2033
1
1.3645
19
1.3645
27
1.3639
31
1.2041
1
1.3601
27
1.3508
46
1.2036
1
  1.2082
1
1.2669
2
  1.3544
3
  1.4531
12
1.4506
12
B3PW91 1.4113
41
1.4451
44
1.4474
44
1.4120
43
1.3544
43
1.3564
44
1.3542
44
1.3483
44
1.3484
41
1.3462
43
1.4636
12
1.1967
1
1.3511
27
1.3532
45
1.3533
43
1.1975
1
1.3596
21
1.3653
20
1.1971
1
  1.2014
1
1.2582
2
1.1973
1
1.3400
3
  1.4408
12
1.4385
12
mPW1PW91 1.4313
40
1.4596
44
1.4645
42
1.4304
42
1.3676
43
1.3731
43
1.3668
44
1.3607
44
1.3655
43
1.3634
42
1.4560
12
1.1919
1
1.3758
27
1.3676
43
1.3602
43
1.1927
1
1.3684
32
1.3717
30
1.1923
1
  1.1962
1
1.2530
2
1.1924
1
1.3343
3
  1.4342
12
1.4318
12
M06-2X 1.4016
30
1.4369
30
1.4253
57
1.3997
30
1.3399
46
1.3463
30
1.3439
30
1.3400
30
1.3405
30
1.3431
31
1.3245
53
1.1877
1
1.3403
28
1.3421
30
1.3449
31
1.1880
1
1.3395
29
1.3438
30
1.1877
1
  1.1909
1
1.2490
2
1.1879
1
1.3320
3
  1.4226
12
1.4219
12
PBEPBE 1.4274
42
1.4719
42
1.4753
40
1.4444
39
1.3748
40
1.3776
41
1.3798
40
1.3719
41
1.3763
42
1.3696
41
1.4300
19
1.2172
1
1.3756
27
1.3774
42
1.3718
41
1.2698
5
1.3706
29
1.3776
31
1.2791
4
  1.2226
1
1.2789
2
1.2183
1
1.3631
3
1.2110
1
1.4693
12
1.4675
12
PBEPBEultrafine 1.3009
1
1.4971
17
1.3210
1
1.2747
1
1.3898
34
1.3963
18
1.3928
18
1.3917
18
1.2200
1
1.5057
12
1.4971
12
1.2172
1
1.3800
19
1.3799
27
1.3773
27
1.2187
1
1.3734
27
1.3724
27
1.2179
1
  1.2227
1
1.2789
2
  1.3631
3
  1.4703
12
1.4679
12
PBE1PBE 1.4094
26
1.4427
25
1.4439
26
1.4092
26
1.3460
42
1.3531
26
1.3507
26
1.3454
26
1.3460
26
1.3458
26
1.4546
12
1.1916
1
1.3488
26
1.3484
26
1.3470
26
1.1925
1
1.3440
26
1.3432
26
1.1921
1
  1.1957
1
1.2523
2
1.1923
1
1.3325
3
  1.4295
12
1.4279
12
HSEh1PBE 1.4121
29
1.4344
45
1.4448
29
1.4091
29
1.3451
45
1.3520
29
1.3426
45
1.3439
29
1.3444
29
1.3447
29
1.4560
12
1.1918
1
1.3455
27
1.3468
29
1.3381
45
1.1927
1
1.3438
28
1.3433
28
1.1922
1
  1.1961
1
1.2529
2
1.1924
1
1.3344
3
  1.4306
12
1.4290
12
TPSSh 1.4416
17
1.4652
19
1.4679
19
1.4331
19
1.3549
49
1.3733
19
1.3558
48
1.3659
19
1.3835
17
1.3575
50
1.4827
12
  1.3692
19
1.3615
20
1.3522
48
1.3846
16
1.3631
19
1.3621
19
1.3762
16
  1.2120
1
1.2711
2
1.2077
1
1.3575
3
  1.4584
12
1.4555
12
wB97X-D 1.4288
17
1.4559
17
1.4146
58
1.4228
17
1.3397
58
1.3676
17
1.3384
58
1.3606
17
1.3352
58
1.3606
17
1.4550
12
  1.3365
58
1.3388
58
1.3344
58
1.3580
16
1.3565
17
1.3324
58
1.3513
16
  1.1983
1
1.1961
1
1.1941
1
1.4970
1
  1.4359
12
1.4336
12
B97D3 1.4596
16
1.4320
57
1.5045
16
1.4772
16
1.3666
57
1.4089
16
1.3660
57
1.4045
16
1.3601
58
1.4001
16
1.3692
58
  1.3666
57
1.4084
16
1.3597
56
1.3976
16
1.3962
16
1.3785
57
1.3867
16
  1.2188
1
1.2157
1
1.2137
1
1.5377
1
  1.4698
12
1.3686
52
STO-3G 3-21G 3-21G* 6-31G 6-31G* 6-31G** 6-31+G** 6-311G* 6-311G** 6-31G(2df,p) 6-311+G(3df,2p) 6-311+G(3df,2pd) TZVP cc-pVDZ cc-pVTZ cc-pVQZ aug-cc-pVDZ aug-cc-pVTZ aug-cc-pVQZ cc-pV(T+d)Z cc-pCVDZ cc-pCVTZ cc-pCVQZ aug-cc-pCVTZ Sadlej_pVTZ daug-cc-pVDZ daug-cc-pVTZ
Moller Plesset perturbation MP2 1.4242
40
1.4764
41
1.4786
41
1.4687
37
1.3797
55
1.4142
44
1.4125
45
1.3712
53
1.3927
44
1.3934
43
1.4870
12
1.2184
1
1.3874
51
1.3997
45
1.3897
43
1.3874
27
1.4037
40
1.4018
36
1.3274
21
1.2664
1
1.3168
4
1.3104
5
1.2187
1
1.3627
3
1.2155
1
1.4703
12
1.4826
11
MP2=FULL 1.4227
40
1.4738
39
1.4796
37
1.4708
34
1.4123
44
1.4131
44
1.4122
45
1.3867
45
1.3885
42
1.3931
41
1.4827
12
1.2156
1
1.4092
27
1.4013
43
1.3899
41
1.3869
28
1.3994
32
1.3882
36
1.3955
23
1.2613
1
1.3159
4
1.3083
5
1.2164
1
1.3603
3
1.2150
1
1.4690
12
1.4650
12
ROMP2 1.3659
13
1.4298
13
1.4298
13
1.4122
14
1.3104
15
1.3099
15
1.3121
15
1.2843
15
1.2852
15
1.2880
15
1.2851
4
1.2458
1
1.2797
8
1.2922
15
1.2893
15
1.2285
2
1.3004
14
1.2349
4
1.2358
3
  1.2537
1
1.2803
2
  1.2822
2
  1.3232
5
1.3124
5
MP3 1.2929
1
1.2997
1
1.2997
1
1.2510
1
1.3907
38
1.2113
1
1.3706
46
1.1909
1
1.1909
1
1.1942
1
1.4925
9
1.1887
1
1.4001
16
1.4017
16
1.3938
16
1.1879
1
1.2013
1
1.1926
1
1.1880
1
  1.1974
1
1.2517
2
1.1876
1
1.3410
3
  1.4962
9
1.4854
9
MP3=FULL   1.5541
13
1.5560
13
1.5263
13
1.3669
47
1.4578
13
1.3660
47
1.4348
13
1.4356
13
1.4365
13
1.5268
8
  1.3991
16
1.4009
16
1.3895
16
  1.4472
13
1.4364
12
    1.1968
1
1.2500
2
1.1858
1
1.3389
3
  1.4949
9
1.4791
9
MP4 1.2851
1
1.4914
29
1.4464
3
1.4319
3
1.4235
37
1.2905
2
1.2930
2
1.3168
5
1.4105
32
1.2222
1
1.5551
8
1.2208
1
1.4330
23
1.4255
23
1.4033
32
1.2216
1
1.4312
23
1.4241
22
1.2216
1
  1.2271
1
1.2766
2
1.2215
1
1.3766
3
  1.5532
9
1.5448
9
MP4=FULL 1.2851
1
1.4791
18
1.3618
1
1.3140
1
1.4384
23
1.2396
1
1.2416
1
1.2219
1
1.4167
23
1.2207
1
1.5504
8
1.2182
1
  1.4247
23
1.4164
23
1.2194
1
1.4303
23
1.4167
21
1.2193
1
  1.2265
1
1.2243
1
1.2198
1
1.3748
3
  1.5515
9
1.5345
9
B2PLYP 1.4381
19
1.4870
19
1.4896
19
1.4505
19
1.3666
38
1.3804
19
1.3709
21
1.3672
19
1.3652
20
1.3711
22
1.4406
10
1.2118
1
1.3743
19
1.3719
19
1.3556
41
1.2126
1
1.3704
19
1.3706
22
1.2123
1
  1.2175
1
1.2715
2
1.2123
1
1.3656
3
  1.4656
12
1.4654
13
B2PLYP=FULL 1.4381
19
1.4580
21
1.4895
19
1.4505
19
1.3686
21
1.3801
19
1.3671
21
1.3668
19
1.3673
19
1.3686
19
1.4826
12
  1.3740
19
1.3717
19
1.3699
19
  1.3703
19
1.3651
19
    1.2173
1
1.2710
2
1.2117
1
1.3649
3
  1.4653
12
1.4591
12
B2PLYP=FULLultrafine 1.4502
17
1.5023
17
1.5052
17
1.4686
17
1.3668
55
1.3968
17
1.3955
17
1.3836
17
1.3841
17
1.3854
17
1.4826
12
  1.3910
17
1.3576
53
1.3552
54
  1.3866
17
1.3531
54
    1.2173
1
1.2143
1
  1.5053
3
  1.4653
12
1.4591
12
Configuration interaction CID 1.2779
1
1.4700
40
1.4707
40
1.4433
39
1.3739
44
1.2647
2
1.2652
2
1.3573
41
1.1861
1
1.1875
1
1.4393
12
1.1823
1
1.1917
1
1.4510
13
1.4376
13
1.1810
1
1.1952
1
1.1858
1
1.1810
1
  1.1921
1
1.2454
2
1.1807
1
1.3299
3
  1.4335
12
1.4209
12
CISD 1.2781
1
1.4793
41
1.4799
41
1.4544
40
1.3778
44
1.3293
1
1.2696
2
1.3615
41
1.1898
1
1.1908
1
1.4439
12
1.1859
1
1.1957
1
1.4570
13
1.4422
13
1.1846
1
1.1987
1
1.1893
1
1.1846
1
  1.1957
1
1.2493
2
1.1843
1
1.3347
3
  1.4385
12
1.4250
12
STO-3G 3-21G 3-21G* 6-31G 6-31G* 6-31G** 6-31+G** 6-311G* 6-311G** 6-31G(2df,p) 6-311+G(3df,2p) 6-311+G(3df,2pd) TZVP cc-pVDZ cc-pVTZ cc-pVQZ aug-cc-pVDZ aug-cc-pVTZ aug-cc-pVQZ cc-pV(T+d)Z cc-pCVDZ cc-pCVTZ cc-pCVQZ aug-cc-pCVTZ Sadlej_pVTZ daug-cc-pVDZ daug-cc-pVTZ
Quadratic configuration interaction QCISD 1.4173
3
1.4915
38
1.4911
35
1.4968
40
1.4033
42
1.4097
42
1.4035
42
1.3875
42
1.3839
39
1.3827
39
1.4420
14
1.1995
1
1.4012
27
1.3940
43
1.3723
37
1.1985
1
1.4040
30
1.3931
31
1.1986
1
  1.2080
1
1.2680
2
1.1982
1
1.3589
3
  1.4772
11
1.4627
11
QCISD(T) 1.2855
1
1.4049
2
1.4049
2
1.2850
1
1.4266
40
1.2918
2
1.3469
5
1.4461
17
1.3681
2
1.2125
1
1.4645
13
1.2093
1
1.4175
26
1.4056
35
1.3941
35
1.2090
1
1.4047
30
1.3988
24
1.2091
1
  1.2165
1
1.2757
2
  1.3770
3
  1.5070
11
1.4849
11
QCISD(T)=FULL         1.4681
17
  1.4667
17
      1.5330
9
    1.4581
17
1.4366
17
1.4342
16
1.4567
17
1.4334
16
1.3899
14
  1.2159
1
1.2739
2
  1.4559
2
  1.5084
10
1.4797
10
QCISD(TQ)   1.3334
1
1.3334
1
1.2864
1
1.4371
9
1.2311
1
1.3441
9
1.2107
1
1.2107
1
1.2130
1
1.5521
3
1.2089
1
  1.3326
9
1.3126
9
1.2765
6
1.4281
9
1.2828
6
1.2796
2
               
QCISD(TQ)=FULL         1.3416
9
  1.3570
8
            1.3464
8
1.3199
8
1.3030
3
1.3348
9
1.2768
6
1.3475
1
               
Coupled Cluster CCD 1.4149
3
1.4868
41
1.4881
41
1.4663
41
1.3885
44
1.3937
41
1.3920
41
1.3738
42
1.3698
39
1.3706
39
1.4237
14
1.1919
1
1.3865
27
1.3855
41
1.3670
36
1.1908
1
1.3782
35
1.3754
32
1.1909
1
  1.2015
1
1.2540
2
1.1905
1
1.3439
3
  1.4457
12
1.4333
12
CCSD 1.2843
1
1.3871
2
1.3871
2
1.3550
2
1.3918
41
1.4907
13
1.4628
16
1.4401
15
1.4803
14
1.4139
21
1.4362
14
1.1976
1
1.3970
27
1.3967
31
1.3888
32
1.3799
23
1.4001
27
1.3861
29
1.3699
19
  1.2063
1
1.2632
2
1.1963
1
1.3570
3
  1.4633
12
1.4472
12
CCSD=FULL 1.2843
1
1.3169
1
1.3169
1
1.2708
1
1.4076
29
1.2194
1
1.2207
1
1.1992
1
1.1992
1
1.4123
21
1.4645
12
1.1951
1
1.3959
27
1.3985
27
1.3846
29
1.3767
23
1.3993
27
1.3803
29
1.3310
17
  1.2056
1
1.2612
2
1.1943
1
1.3545
3
  1.4681
11
1.4493
11
CCSD(T) 1.2856
1
  1.3314
1
1.2834
1
1.4147
41
1.4574
17
1.4821
16
1.4529
16
1.4519
16
1.4454
15
1.4563
13
1.2083
1
1.4177
26
1.4104
36
1.3855
37
1.3889
23
1.4043
34
1.4001
25
1.3559
18
  1.3134
4
1.3109
5
  1.3752
3
  1.4868
12
1.4717
12
CCSD(T)=FULL 1.2856
1
1.3313
1
1.3313
1
1.2833
1
1.4309
32
1.2287
1
1.2304
1
1.2090
1
1.2090
1
1.2103
1
1.4869
11
1.2058
1
1.4204
25
1.3979
27
1.3971
28
1.3892
22
1.4144
29
1.3918
23
1.3547
17
  1.3126
4
1.3089
5
1.2058
1
1.3729
3
  1.4886
11
1.4634
12
STO-3G 3-21G 3-21G* 6-31G 6-31G* 6-31G** 6-31+G** 6-311G* 6-311G** 6-31G(2df,p) 6-311+G(3df,2p) 6-311+G(3df,2pd) TZVP cc-pVDZ cc-pVTZ cc-pVQZ aug-cc-pVDZ aug-cc-pVTZ aug-cc-pVQZ cc-pV(T+d)Z cc-pCVDZ cc-pCVTZ cc-pCVQZ aug-cc-pCVTZ Sadlej_pVTZ daug-cc-pVDZ daug-cc-pVTZ

average calculated bond lengths (Å)
Methods with effective core potentials (select basis sets)
CEP-31G CEP-31G* CEP-121G CEP-121G* LANL2DZ SDD cc-pVTZ-PP aug-cc-pVTZ-PP Def2TZVPP
hartree fock HF 1.3856
42
1.3386
43
1.3843
42
1.3359
43
1.3833
42
1.3833
42
    1.3087
58
ROHF 1.2167
1
1.1844
1
1.2144
1
1.1791
1
1.2073
1
1.2072
1
    1.2263
3
density functional LSDA 1.2411
2
1.1968
2
1.2373
2
1.1896
2
1.2221
2
1.2219
2
     
BLYP 1.3038
1
1.2576
1
1.3008
1
1.2529
1
1.2996
1
1.2996
1
    1.5275
12
B1B95 1.3727
11
1.3147
12
1.2122
2
1.1689
2
1.2056
2
1.2056
2
    1.4735
11
B3LYP 1.4318
42
1.3864
43
1.4298
42
1.3831
43
1.4295
41
1.4258
42
    1.3463
58
B3LYPultrafine 1.2757
1
1.2329
1
1.2728
1
1.2282
1
1.2684
1
1.2684
1
    1.4824
12
B3PW91 1.2666
1
1.2243
1
1.2636
1
1.2192
1
1.2603
1
1.2602
1
    1.4693
12
mPW1PW91 1.2601
1
1.2188
1
1.2571
1
1.2137
1
1.2539
1
1.2538
1
    1.4616
12
M06-2X 1.2468
1
1.2089
1
1.2431
1
1.2023
1
1.2468
1
1.2466
1
    1.4525
12
PBEPBE 1.2900
1
1.2437
1
1.2870
1
1.2386
1
1.2872
1
1.2871
1
    1.3545
57
PBEPBEultrafine 1.2900
1
1.2437
1
1.2870
1
1.2387
1
1.2872
1
1.2871
1
    1.5038
12
PBE1PBE 1.2595
1
1.2180
1
1.2565
1
1.2128
1
1.2539
1
1.2538
1
    1.4603
12
HSEh1PBE 1.2608
1
1.2188
1
1.2577
1
1.2137
1
1.2547
1
1.2546
1
    1.4617
12
TPSSh                 1.4886
12
wB97X-D 1.4299
17
1.3837
17
1.4274
17
1.3801
17
1.4231
17
1.4233
17
    1.4601
12
B97D3                 1.5034
12
Moller Plesset perturbation MP2 1.4700
40
1.4330
43
1.4620
39
1.4291
43
1.4712
38
1.4691
38
    1.3659
58
MP2=FULL 1.3513
1
1.2637
1
1.3447
1
1.2587
1
1.3557
1
1.3554
1
    1.4949
12
ROMP2 1.4249
1
1.2934
1
1.4186
1
1.2895
1
1.4484
1
1.4485
1
    1.3045
3
MP3 1.2789
1
1.2307
1
1.2698
1
1.2236
1
1.2673
1
1.2667
1
    1.5361
8
MP3=FULL                 1.5334
8
MP4 1.3184
1
1.2559
1
1.3183
1
1.2529
1
1.3244
1
1.3246
1
    1.6132
7
MP4=FULL 1.3184
1
1.2559
1
1.3183
1
1.2529
1
1.3243
1
1.3244
1
    1.6095
7
B2PLYP 1.3003
1
1.2445
1
1.2970
1
1.2400
1
1.2972
1
1.2971
1
    1.4900
12
B2PLYP=FULL                 1.4892
12
B2PLYP=FULLultrafine                 1.4892
12
Configuration interaction CID 1.2754
1
1.2235
1
1.2679
1
1.2169
1
1.2663
1
1.2659
1
    1.4707
11
CISD 1.2801
1
1.2268
1
1.2729
1
1.2204
1
1.2719
1
1.2714
1
    1.4753
11
Quadratic configuration interaction QCISD 1.2957
1
1.2401
1
1.2896
1
1.2344
1
1.2901
1
1.2898
1
    1.5084
11
QCISD(T) 1.3029
1
1.2478
1
1.2977
1
1.2428
1
1.2993
1
1.2990
1
    1.5390
10
QCISD(T)=FULL                 1.5442
9
QCISD(TQ) 1.3066
1
1.2498
1
1.3004
1
1.2441
1
1.3025
1
1.3021
1
     
Coupled Cluster CCD 1.2871
1
1.2334
1
1.2801
1
1.2273
1
1.2798
1
1.2794
1
    1.4878
11
CCSD 1.2932
1
1.2386
1
1.2871
1
1.2327
1
1.2870
1
1.2867
1
    1.5016
11
CCSD=FULL 1.2932
1
1.2386
1
1.2871
1
1.2327
1
1.2869
1
1.2865
1
    1.4987
11
CCSD(T) 1.3019
1
1.2471
1
1.2965
1
1.2420
1
1.2980
1
1.2977
1
    1.5288
10
CCSD(T)=FULL 1.3019
1
1.2471
1
1.2965
1
1.2420
1
1.2979
1
1.2975
1
    1.5257
10
For descriptions of the methods (AM1, HF, MP2, ...) and basis sets (3-21G, 3-21G*, 6-31G, ...) see the glossary in section I.C. Predefined means the basis set used is determined by the method.