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Thermochemistry (Read 3146 times)
Gerrit-Jan Linker
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Posts: 75
01.05.12 at 14:58:01
Recipe to get to the Gibbs free energy.
  • Etotal: total electronic energy
    Calculate Etotal: the total electronic energy. As calculated by Molcas or any other QC package irrespective the choice of Hamiltonian (HF or DFT, e.g.)
    E.g. for H2 this is E(RHF) =  -1.12682782541 hartree
  • ZPVE: Zero point vibrational energy
    Calculate the vibrational frequencies ν.
    EZPVE = ∑½hcνi where the sum runs over all frequencies i.
    With ½hc = 2.27827*10-6 cm hartree, we get ZPVE=0.010585 hartree/particle
  • E0: the zero point corrected total energy.
    Using the above calculated numbers for Etotal and EZPVE we get E0= -1.116243 hartree
  • E0-298: Thermal correction to the energy at 298.15K
    E(0-298) = dEel + dEvib + dErot + dEtrans
    dEel: Contributions of electronic excited states
         Usually excited states very far from the ground state and this term is neglected.
    dEvib: Contributions from vibrational degrees of freedom
        dEvib=kB∑½Θi[e^(Θi/T) -1]
         where Θi=hcνi/kB is called the vibrational temperature (K)
    dErot: Constributions from rotational degrees of freedom
        An approximate formula for the rotational contribution is dErot=RT
        At T=298.15K this is 0.5925 kcal/mol
    dEtrans: Constributions from translational degrees of freedom
        The tranlsational of an ideal gas is dEtrans = 3/2 RT.
  • E298=E0 and E0-298
  • H298: the enthalpy at 298.15K
    H298=E298 + pv = E298 + RT (only being valid for molar quantities of an ideal gas)
  • G298: the free enthalpy or Gibbs free energy at 298.15K
    G298=H298 -T*S298
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    « Last Edit: 01.05.12 at 15:28:06 by Gerrit-Jan Linker »  

    Gerrit-Jan Linker
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