The optimization is done in the Gaussian software and we calculate the energy. Can the energy of optimized structure be considered as zero point energy of the molecule?
Dear Xavier,
Zero-point energy is the lowest possible energy that a quantum mechanical physical system may have; it is the energy of its ground state. All quantum mechanical systems undergo fluctuations even in their ground state and have an associated zero-point energy, a consequence of their wave-like nature. The uncertainty principle requires every physical system to have a zero-point energy greater than the minimum of its classical potential well. This results in motion even at absolute zero. Therefore, instead, the lowest-energy state (the ground state) of the system must have a distribution in position and momentum that satisfies the uncertainty principle which implies its energy must be greater than the minimum of the potential well.
Hoping this will be helpful,
Rafik
Dear Xavier,
Zero-point energy is the lowest possible energy that a quantum mechanical physical system may have; it is the energy of its ground state. All quantum mechanical systems undergo fluctuations even in their ground state and have an associated zero-point energy, a consequence of their wave-like nature. The uncertainty principle requires every physical system to have a zero-point energy greater than the minimum of its classical potential well. This results in motion even at absolute zero. Therefore, instead, the lowest-energy state (the ground state) of the system must have a distribution in position and momentum that satisfies the uncertainty principle which implies its energy must be greater than the minimum of the potential well.
Hoping this will be helpful,
Rafik
Dear Xavier, If you want to calculate the zero-point energy of your molecular system, in addition to the geometry optimization, yo should perform a frequency calculation (freq keyword in Gaussian). You can check an example of this kind of calculation in the following link:
http://www.gaussian.com/g_tech/g_ur/k_freq.htm
Good luck!
Matias
respected sir,
no definately zpe and energy are not the same . if u r using gaussian then the output file gives u zpe and entropy but the result u get by viewing in result column gives u energy both r diffrent . send me a sample of ur file so that i can see both the values. hope that helps
Addition to Matias.
Total ground state energy is energy of optimization plus zero point energy.
The zero-point energy that you calculate in Gaussian is a thermal property, more influenced by the relative positions of the nuclei. The total electronic energy (energy of optimization) is calculated by considering explicitly the interactions among electrons. You shouldn't get them confused.
Eugene,
You are correct. I guess using the word "thermal" would be a misnomer... I meant that it requires calculating the vibrational frequencies.
A la Gaussian: "Zero-point vibrational energy: ..."
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