Solvation models, Polarizable Continuum Model (PCM)

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Calculation of solvent effects

Some tests performed on some organic and inorganic species, show that using the default settings (for Gaussian03 defaults + SPHEREONH) the relative energies of a system are on average around 1.5 kcal/mol different between one and the other version of the program. The differences found range between 0.8 and 3.9 kcal/mol.

Some of the species tested are:

                       g09    g03		
([Fe]-H2+) + N(CH3)3        0	  0
([Fe]-H2+) + N(CH3)3     -1.7	-0.6
([Fe]-H-H-N(CH3)3+)      -2.6	 1.3
([Fe]-H) + (+H-N(CH3)3)   5.0	 8.1
----
(BF4-) + N(CH3)3 + (Fe-H2+)	    0      0
(BF4-N(CH3)3-) + (Fe-H2+)         -3.3    -1.9
([Fe]-H2···BF4) + (N(CH3)3)	  -13.3	   -12.1
([Fe]-H2··BF4··N(CH3)3)	  -17.3	   -14.7
([Fe]-H-H-N(CH3)3··BF4)	  -19.7    -16.2
(BF4-···H-N(CH3)3) + (Fe-H2+)    -18.1	   -15.8

energies in kcal/mol.[Fe] is an iron transition metal complex.

According to these results, at least for some systems, the relative energies are significantly different.

Optimization in solvent

• Optimized structures in gas phase of 300-400 basis sets can be re-optimized in solvent in a period of 7 hours- 1 day at cesca (cadi nodes). In some cases the optimization is achieved in few cycles (16), in most cases between 20-40 but and in very few other cases up to 500 cycles are needed.

• Frequency calculations on these structures can be performed in a period of around 3 hours at cadi.

• The relative energies in kcal/mol of some of the single points (SP) and optimized species (OPT) are:

 (in kcal/mol)                    SP      OPT  
(BF4-) + N(CH3)3 + (Fe-H2+)         0       0
(BF4-N(CH3)3-) + (Fe-H2+)         -3.3     -3.7
([Fe]-H2···BF4) + (N(CH3)3)      -13.3	   -15.4
([Fe]-H2··BF4··N(CH3)3)          -17.3	   -19.3
([Fe]-H-H-N(CH3)3··BF4)          -19.7    -21.5
(BF4-···H-N(CH3)3) + (Fe-H2+)    -18.1	   -20.6

Over the tests performed, the difference between the relative energies of the optimized species and the relative energies of the single points are in average: 0.6 kcal/mol.The differences range between +1.1 to -2.5 kcal/mol.

• Optimizations without previous gas phase optimization (from un-optimized species), need aprox. the same number of cycles to optimize it in gas phase or in solvent. For a system with 560 basis sets, 143 cycles, 8 days + 18 hours in cadi are needed for the optimization in solvent and 141 cycles, 7 days and 11 hours also in cadi for the optimization in gas phase.

This is a 17% more time for the optimization in solution.