Dear Tanmay Sarkar,

Using the aug-cc-pVTZ basis set with much diffuse exponents to optimize the structure of methyl iodide is meaningless. The def2- or SDD basis set without any diffuse exponents on hydrogen atoms is recommended. The keywords for this basis sets will be determined by the software you selected. Please read their manual carefully.

Hope this helps.

Guanghao Qu

I suppose that the author of the question means to use different BSs for C, H, and that the aug-CC-pVTZ-PP set is intended to be adopted for iodine only. If this is the case, the choice of the aug-cc-pVTZ BS may not be useless, especially if the results from the calculations on CH3I are meant to be compared with others where a larger charge separation is possibly involved.

Apart from these considerations, the basis set can be obtained from the Basis Set Exchange web site (https://www.basissetexchange.org). In the specific case of Gaussian, a very popular commercial suite, the BS and related ECPs can be imported with the gen and pseudo=read keywords. In this case, the aug-cc-pVTZ is to be added at the end of the input file as follows:

!---------------------------------------------------------------------- ! Basis set: aug-cc-pVTZ-PP !---------------------------------------------------------------------- ! I 0 S 11 1.00 5.546500D+03 1.560000D-04 8.382140D+02 9.860000D-04 1.821870D+02 2.792000D-03 3.121230D+01 -4.325100D-02 1.953140D+01 2.341340D-01 8.240990D+00 -7.509430D-01 2.194550D+00 8.829680D-01 1.109110D+00 4.620610D-01 3.746410D-01 2.228600D-02 1.770800D-01 -4.353000D-03 8.106100D-02 1.102000D-03 S 1 1.00 3.746410D-01 1.000000D+00 S 1 1.00 1.770800D-01 1.000000D+00 S 11 1.00 5.546500D+03 -7.300000D-05 8.382140D+02 -5.080000D-04 1.821870D+02 -1.158000D-03 3.121230D+01 1.219300D-02 1.953140D+01 -8.785400D-02 8.240990D+00 3.382000D-01 2.194550D+00 -5.765500D-01 1.109110D+00 -4.092980D-01 3.746410D-01 5.674590D-01 1.770800D-01 6.124890D-01 8.106100D-02 1.432310D-01 S 1 1.00 1.001000D-01 1.000000D+00 S 1 1.00 4.120000D-02 1.0000000 P 9 1.00 1.889880D+02 5.850000D-04 2.128680D+01 3.692300D-02 1.003960D+01 -2.353240D-01 3.451800D+00 3.414830D-01 1.974560D+00 5.347880D-01 1.024200D+00 2.651410D-01 4.494370D-01 2.578700D-02 1.866480D-01 5.220000D-04 7.348100D-02 6.060000D-04 P 1 1.00 5.981000D-01 1.000000D+00 P 9 1.00 1.889880D+02 -2.560000D-04 2.128680D+01 -1.168200D-02 1.003960D+01 8.319200D-02 3.451800D+00 -1.569700D-01 1.974560D+00 -2.245180D-01 1.024200D+00 -1.144510D-01 4.494370D-01 3.753560D-01 1.866480D-01 5.751360D-01 7.348100D-02 2.459170D-01 P 1 1.00 9.618000D-02 1.000000D+00 P 1 1.00 3.590000D-02 1.0000000 D 9 1.00 1.326620D+02 5.720000D-04 3.760540D+01 4.402000D-03 1.038910D+01 -4.092200D-02 6.490170D+00 9.966100D-02 3.454510D+00 3.226630D-01 1.844130D+00 4.003430D-01 9.624780D-01 2.683060D-01 4.728530D-01 8.484700D-02 1.932000D-01 7.632000D-03 D 1 1.00 4.728530D-01 1.000000D+00 D 1 1.00 1.932000D-01 1.000000D+00 D 1 1.00 7.820000D-02 1.0000000 F 1 1.00 4.064000D-01 1.000000D+00 F 1 1.00 1.848000D-01 1.0000000 **** I 0 I-ECP 4 28 g potential 1 2 1.00000000 0.00000000 s-g potential 3 2 40.03337600 49.98964900 2 17.30057600 281.00655600 2 8.85172000 61.41673900 p-g potential 4 2 15.72014100 67.41623900 2 15.20822200 134.80769600 2 8.29418600 14.56654800 2 7.75394900 28.96842200 d-g potential 4 2 13.81775100 35.53875600 2 13.58780500 53.33975900 2 6.94763000 9.71646600 2 6.96009900 14.97750000 f-g potential 4 2 18.52295000 -20.17661800 2 18.25103500 -26.08807700 2 7.55790100 -0.22043400 2 7.59740400 -0.22164600

Remember to insert an empty line at the end of the file. I hope this may help.

Although for iodine atom, it is not need to use aug-CC-pVTZ basis set in the geometry optimization. The structure of local energy minimum in the potential energy surface can be finded by normal basis sets without diffuse function, such as def2-TZVP. This will save much computing time.

If you want to calculate the single point energy or electronic structure after geometry optimization, you can use high precise basis sets with diffuse function.