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Fabian Weber
fabian.weber@mars.uni-freiburg.de

> What will it give to me?
Imagine you have a diatomic molecule. You probably know, that the minimum energy is found in a Morse potential (http://en.wikipedia.org/wiki/Morse_potential). The Morse potential can be approximated as a harmonic oscillator around the minimum (http://en.wikipedia.org/wiki/Quantum_harmonic_oscillator). If you regard a harmonic oscillator with quantum mechanical equations you'll find out, that the Molecule can never reach the minimum of your curve. The difference between the lowest possible state and the minimum of your PES-curve is the zero point energy. �


>Perhaps, it is necessary to correct received for them TOTAL ENERGY?
If you compare energies you often compare your TOTAL ENERGYs and your corrected TOTAL ENERGYs, where you added the zero point energy. Note, that your TOTAL ENERGY is always negative, while your zero point energy is positive, so you get:
-1311.5429273+0.280207=-1311.26272

Your corrected total energies should be considered to be more precise than the not corrected ones, but always remember that the zero point energy is approximated.

hope this helps,

Fabian





On Thu Jan 20 '11 0:15am, Alexandr wrote
----------------------------------------
>Hi,

>I perform work with a molecule of organic connection from 40 atoms.
>In the beginning I find a stationary point:

>$contrl=optimize [.]
>[...]

>... I receive:
>[...]
>ENERGY =-1311.5429273
>[...]
>MAXIMUM GRADIENT = 0.0000744 RMS GRADIENT = 0.0000248
>1 ***** EQUILIBRIUM GEOMETRY LOCATED *****
>[...]

>Further for the found stationary point I calculate hessian:
>$contrl=hessian [...]
>[...]
> �� � � � � � � � � � ��1 � � � �2 � � � 3 � � �4 � � �5
> � � � FREQUENCY: � �36.36 � �32.43 � 10.77 � 8.51 � 5.90 �
> � �REDUCED MASS: � 8.77061 8.84577 9.28704 8.91738 9.52289
> � �IR INTENSITY: � 0.18500 0.10810 0.05133 0.12040 0.03344
>[...}
>Thank God, all frequencies are positive, hence, the found stationary point
>characterises a minimum (local) on PES.
>But further I read in thermodynamics section:
>[...}
> THE HARMONIC ZERO POINT ENERGY IS (SCALED BY 1.000)
> � � � �0.280207 HARTREE/MOLECULE
>[...}
>How to use this size 0.280207? To take away, add to size TOTAL ENERGY received above?
> What will it give to me? I count a series from this and ionic forms.
>Perhaps, it is necessary to correct received for them TOTAL ENERGY?
>I wait for the help.

>With regards,
>Alexandr
>