Question

I am trying to find the enthalpies of formation for a couple organometallics in ORCA and as a first time user I am a bit bogged down in the terminology. The Gaussian manual has a section on these calculations (4.3 on page 14) and I am trying to confirm whether the total energy of the molecules/atoms is the same as the inner energy or total thermal energy you can get from the ORCA FREQ calculations.

Answer 1

Not same. In ORCA, the total energy of a molecule or atom is not the same as the internal energy or total thermal energy obtained from ORCA FREQ calculations. The total energy refers to the sum of the electronic energy and the nuclear repulsion energy of the system. It does not include the contributions from thermal motion or entropy.

Step-by-step explanation:

On the other hand, the internal energy, or total thermal energy, obtained from ORCA FREQ calculations includes the contributions from both the electronic energy and the thermal motion of the atoms in the system. It takes into account the effects of temperature and the distribution of energy among different degrees of freedom.

To calculate the enthalpy of formation for organometallics in ORCA, you would need to consider the total energy of the system, including any necessary corrections for thermal effects. It is important to carefully follow the methodology and procedures outlined in the Gaussian manual, specifically section 4.3 on page 14, to ensure accurate and reliable results.

If you are still unsure about the terminology and calculations, it would be beneficial to consult additional resources, such as textbooks or scientific papers, or seek guidance from an expert in the field of organometallic chemistry or computational chemistry.