Calculating delta G from frequency calculation.


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Hello,
I am trying to calculate the delta G of a molecule and am having trouble figuring out what values I need to use to do so at a given temperature. I run a geometry optimization followed by a frequency calculation and I get this section in the output:
From the projected analysis 
The Zero-Point Energy (Kcal/mol) =          28.51472262

 center of mass
 --------------
 x =   0.00000163 y =   0.00000203 z =   0.00000051

 moments of inertia (a.u.)
 ------------------
          11.468747259680           0.000017667545          -0.000007476804
           0.000017667545          11.468763189783           0.000011061644
          -0.000007476804           0.000011061644          11.468783251860

 Rotational Constants
 --------------------
 A=   5.249070 cm-1  (  7.552071 K)
 B=   5.249051 cm-1  (  7.552045 K)
 C=   5.249044 cm-1  (  7.552035 K)


 Temperature                      =   298.15K
 frequency scaling parameter      =   1.0000

 Zero-Point correction to Energy  =   28.493 kcal/mol  (  0.045406 au)
 Thermal correction to Energy     =   30.287 kcal/mol  (  0.048266 au)
 Thermal correction to Enthalpy   =   30.880 kcal/mol  (  0.049210 au)

 Total Entropy                    =   49.384 cal/mol-K
   - Translational                =   34.246 cal/mol-K (mol. weight =  16.0313)
   - Rotational                   =   15.068 cal/mol-K (symmetry #  =        1)
   - Vibrational                  =    0.070 cal/mol-K

 Cv (constant volume heat capacity) =    6.384 cal/mol-K
   - Translational                  =    2.979 cal/mol-K
   - Rotational                     =    2.979 cal/mol-K
   - Vibrational                    =    0.425 cal/mol-K

What values do I need to use in my calculation? Is the thermal correction to Enthalpy the Enthalpy or do I need to add/subtract it from something?

Thank you in advance.

Forum Vet
The thermal correction to enthalpy should get you to Delta H. To get to Delta G you need Delta H - T Delta S

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So then my Delta G with the posted data is 30.880 - T* 49.384? I just want to make sure I am using the right values.

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Quote:Beckmw May 13th 3:29 pm
So then my Delta G with the posted data is 30.880 - T* 49.384? I just want to make sure I am using the right values.


yes but make sure you are using the same energy units!! deltaH is kcal/mol, and deltaS is cal/mol.K

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Quote:DouDou May 13th 3:45 pm
Quote:Beckmw May 13th 3:29 pm
So then my Delta G with the posted data is 30.880 - T* 49.384? I just want to make sure I am using the right values.


yes but make sure you are using the same energy units!! deltaH is kcal/mol, and deltaS is cal/mol.K

Oh wow good catch! Thank you!


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