I was asked to do OPT in Gaussian with different temperatures. Previously, I did not take temperature into account. And I used the following command.
#p MP2/6-31G* opt
Afterwards, I added temperature at the end of command as below.
#p MP2/6-31G* opt temperature=350
I also tried to set the temperature to 270. But the three computing results had the same conformation and potential energy. Did I do something wrong?
Hai Xiaoquan
With an anxiety, i did test calculations on CH4 with B3LYP/6-31G(d) level of theory at three different temperatures such as 0, 300, 2500 K. However, the three geometries at different temperatures landed at the same geometry on the potential energy surface. But,i found that the temperature corrections are added in the thermo chemistry analysis.
Potential energy surface do not depend on temperature. Optimal geometry do not depend on temeperature. Please, read your lecture notes on quantum chemistry again.
What really depends on temperature is population of vibrational levels and molecular dynamics. And thermochemistry also, as enthalpy and entropy are temperature dependent.
Hi Xiaoquan Sun
If you are optimizing any geometry at different-2 temperature then it'll give the same result since optimization doesn't depend on temperature.
Now if you want to look at the temperature effect first you should do optimization at default temperature then at the optimized geometry calculate frequency and add the command for temperature.
# opt MP2/6-31g(d) geom=connectivity
or
# opt freq MP2/6-31g(d) geom=connectivity temperature=350 test
Thanks, guys.
Geometry optimization is locating a stationary point on the potential energy surface. Each structure has its own location on PES. It has nothing to do with temperature.
I realized how stupid the question is.
Hai Guys,
The explanations are very nice and the same are true. But I am wondering that in the real sense why the molecular systems are breaking after reaching a certain temperature limit (i.e melting point of all solids) if they have same geometry at all temperature.
in BO approximation PES - is a potential, subject to which nuclei are moving. If a system has enough kinetic energy (i.e. temperature), than it might be in a point far from minimum (though, even if it doesn't have - see tunnel effect for example). Optimized geometry is just a minimum on a PES, where system has minimal potential energy.
Each structure has its point on PES, even the molecular system is breaking. Well, at that condition, the molecular maybe has very high potential and far away from any minimum.
From the view of thermodynamics, Boltzmann equation is more clear to demonstrate this. If the high potential structure is product, when the energy difference is fixed, the higher the temperature is, the higher the probability high potential structure would occur.
From the view of kinetics, if the high potential structure is transition state, in Arrhenius Equation, the higher temperature is, the bigger the rate constant will be.
So high temperature has positive effective to reach high potential state.
For the reason of knowledge for all, this link explains very good this question:
http://collum.chem.cornell.edu/documents/Gaussian_optimization.pdf
Please see item 3.
The geometry is independent of temperature . Therefore, specifying a
temperature for a plain geometry optimization ( without the Freq keyword) is
nonsensical . For the same reason, an isotopic substitution will not change the
geometry . To calculate the energy of an isotopically substitud structure , the
keyword ReadIsotopes must be included inside the Freq command.
# method/basis set Freq=ReadIsotopes
- Badges
- Science topic
- Similar topics
- Filing