# Calculating Energies and Enthalpies

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 7:41:00 AM PDT - Fri, Jun 24th 2011 Hi, I wish to reconcile how to treat the corrections to the calculated energy in order to report correctly the Zero-point energy and the Enthalpy for a system of hydrogen bonded complexes. Upon doing a geo/opt with MP2/aug-cc-pVDZ for a hydrogen bonded system you get energy X a.u. Then doing a vib freq analysis you get the corrections ... an example: Zero-Point correction to Energy = 89.053 kcal/mol ( 0.141916 au) Thermal correction to Energy = 95.892 kcal/mol ( 0.152813 au) Thermal correction to Enthalpy = 96.484 kcal/mol ( 0.153757 au) Therefore to get the ZPE do you take X + ZPE Correction? And then likewise for the enthalpy you take X + Thermal Correction to Enthalpy? Or is it some other combination of these corrections? Then upon taking the products-reactants would you also have to add a ?PV value to the get the true enthalpy (H) or is that already included in the corrections? I have attempted to determine the correct methodology by searching journal articles and in the online NWChem documentation but to no avail. Any help would be appreciated. Thanks, Janos

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 10:23:03 AM PDT - Tue, Jun 28th 2011 Answer - from Eric Bylaska (personal communication) http://www.nwchem-sw.org/index.php/NWPW#NWPW_Tutorial_3:_using_isodesmic_reaction_energies... The enthalpy of a molecule at 298.15K is sum of the total energy and a thermal correction to the enthalpy. A good estimate for the thermal correction to the enthalpy can be obtained from a frequency calculation, i.e. H = E + Hcorrection Thermodynamic output from a frequency calculation: ...

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