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Beginner's question for OpenMX 3.7
Date: 2017/03/16 23:05
Name: A. Vogt

Dear all,

I started reading up on DFT as part of my masters thesis, but I ran into a few problems trying to apply what I've learned so far in OpenMX.
First off, I don't have a background in physics or chemistry, so I had to teach myself a few central concepts behind the theory. I say this because there may be a few misinterpretations on what I learned.

To start off, I wanted to find out the total energy of an fcc Cu lattice. I used the file for Mn in the work directory as a basis to work on, changed the appropriate parameters and started the calculation, but the result differs considerably from the values reported in literature.
The values I found were around -1600 Hartrees, while the calculation yielded -55 Hartrees. I've tried to read up on what the causes may be, but I came to no conclusion. There are energy contributions to the total energy that were of suitable magnitude, but I have a hard time understanding what they mean.

My questions are what I may have done wrong, and if there's any literature I can use to understand why there's such a big difference in calculated and actual values. I'm using OpenMX version 3.7, which is the most recent one in the Ubuntu repositories.

Thank you all for your time
A. Vogt
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Re: Beginner's question for OpenMX 3.7 ( No.1 )
Date: 2017/03/17 18:57
Name: Artem Pulkin

"I wanted to find out the total energy of an fcc Cu lattice"

The relevant questions are:

What is the origin on your energy scale? What kind of physical system has the total energy equal to zero?

Do you calculate energy per unit of volume, per atom or per something else?

You may find answers in the literature you are referring to.
Re: Beginner's question for OpenMX 3.7 ( No.2 )
Date: 2017/03/18 00:11
Name: T. Ozaki


I guess that your reference comes from an all electron calculation.
The total energy in OpenMX should be much smaller in absolute scale than
that of the all electron reference, since OpenMX uses pseudopotentials
which exclude core electrons having much deeper energies.


Re: Beginner's question for OpenMX 3.7 ( No.3 )
Date: 2017/03/20 21:47
Name: A. Vogt

Dear Mr. Ozaki and Mr. Pulkin,

thank you for your answer. I really did compare the results with an all-electron calculation, and I understand now that the results for the ground state energy are not comparable between the two.

This has helped me a lot already, and I will go on to calculate cohesive energies as a test for myself.

Thank you again for your help.
A. Vogt

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