Using Better Basis Set

After we finished the optimization of BH3 at the B3LYP/3-21G at level of DFT calculation. Theoretically, this basis set is quite small compare to another in Pople basis set family. Therefore we will study the effect of the bigger size of basis set on the electonics sturcture of the BH3 molecule.

Technically, we could use the optimized structure from previous tutorial as the initial structure for the new basis set in this tutorial. So re-calculation again and change the basis set to the 6-31G(d,p) and also add the optional keyword => "SCF=CONVER=9 INTEGRAL=GRID=ULTRAFINE"

Example of Gaussian input file for 6-31G(d,p) calculation.

%chk=TH_bh3_opt_631g_dp.chk

# opt b3lyp/6-31g(d,p) geom=connectivity scf=conver=9

intergral=grid=ultrafine

BH3 OPTIMIZATION

0 1

 B                  0.00000000    0.00000000    0.00000000

 H                  0.00000000    1.19452700    0.00000000

 H                  1.03449100   -0.59726400    0.00000000

 H                 -1.03449100   -0.59726400    0.00000000

 1 2 1.0 3 1.0 4 1.0

Results

The total energy for 3-21G optimized structure is -26.4622643291     A.U.

The total energy for 6-31G(d,p) optimized structure is -26.6153236411     A.U.

Selected structural parameter

Bond length: B-H for 3-21G optimized structure is 1.19453 A

Bond length: B-H for 6-31G(d,p)  optimized structure is 1.19231 A

Bond Angle: H-B-H 3-21G optimized structure is 120 degree

Bond Angle: H-B-H 6-31G(d,p)  optimized structure is 120 degree