# Questions about calculating heats of formation in CH4

Dear all,

I have some questions about calculating heats of formation of CH4.

The method I use is to use 3 simulation box with 4 H atoms 1 C atom and 1 CH4 Molecule respectively. I calculate the energy of of this three boxes and use the following formula to get heats of formation:

E(heat)=Ech4 (the energy of ch4) -Ec (the energy of carbon atom) - Eh (the energy of 4 H atoms)

I can`t get a value with the former literature and I also find that different box sizes will produce different energy value. I`m confused about it. Could you please help me if you have a better way to make calculate.

Thank you so much!

Your reference states were incorrect. E_C should come from graphite and E_H should come from molecular H2, in both cases not single C or H atoms.

Ray

Thank you Ray. I`m trying to recalculate the data.

Ximeng

I’d like to add that heat of formation is a change in enthalpy not in energy. For small molecules and low temperature the difference is negligible , but it is significant for larger systems and higher temperatures.

Michal

Hi Ray,

I use the state you recommend and I get some results. The energy values I got are listed following:
-80 eV for graphite (16 C atoms )
-145.17 eV for H2 (64 H atoms)
-277.49eV for 16 CH4 molecules

and I got the heat of formation is -3.27eV for each CH4 molecule. Unfortunately, the value from literature is -0.69eV (-15.99 kcal/mole)

I don`t know what`s wrong with my script. could you please help me to see what`s wrong with it?

Thank you so mucn!

Ximeng

datagraphite (666 Bytes)

datahydrogen (2.27 KB)

dataMethane (2.8 KB)

in.C (1.48 KB)

in.CH4 (1.54 KB)

in.H (1.18 KB)

logC.lammps (26.2 KB)

logCH4.lammps (16.3 KB)

logH.lammps (31.3 KB)

There may be something wrong with your input script which I don’t have time to go through, but it may be also related to the quality of the force field.