ABINIT Discussion Forums

Hai

I am trying to calculate the total energy of a NO2 gas molecule. As the ABINIT website includes the computation of total energy of H2 molecule which is discussed in Tutorial 1. For doing NO2 energy calculation, I ran the same input file by only changing the atomic co-ordinates, the type of atoms and the znucl as follows:

acell 10 10 10
rprim 1 0 0
0 1 0
0 0 1
ntypat 2
znucl 7 8
natom 3
typat 2 1 2
xangst -1.094 0.463 0.0
0.0 0.0 0.0
1.094 0.463 0.0
ecut 10
kptopt 0
nkpt 1
nstep 10
toldfe 1.0d-6
diemac 2.0
optforces 1

The simulation ran successfully. But I am not sure of what I did is correct, I would like to check whether the values of acell, rprim and xangst I gave are correct for NO2.
When I tried to see the structure using jmol, it showed an error of "rprim not fount in given file, assuming : rprim 1 0 0 0 1 0 0 0 1".

Please give me some suggestions in this regard by looking at the input file.


Regards
Seba

Dear Seba,

Your input is fine.
You need at least to converge ecut, acell (the larger, the better) as in Tutorial 2.
Then you need to optimize you geometry using optcell and ionmov (and some other realted to) variables (See Tutorial 3 part 4)
I would not worry about jmol and rprim since you use the cartesian system.

Read carefully the first 4 tutorials

Cheers

Hai

Thank you for the reply.

Hai

I have some doubts regarding the optimization of the geometry.
For fully relaxing the geometry of NO2 gas molecule, as in tutorial 3 part 4,
optcell 1
ionmov 3
ntime 10
dilatmx 1.05
ecutsm 0.5
kptopt 1
nshiftk 4
shiftk 0.5 0.5 0.5
0.5 0.0 0.0
0.0 0.5 0.0
0.0 0.0 0.5
ngkpt 2 2 2 ... with other input variables will do the structural optimization ??

But in ABINIT website, it is written that atomic position relaxation is done by two steps:
1) with optcell 0 and ionmov 2, the atomic positions are relaxed within a fixed geometry and the "xred" from the output file is given as "xred" for the input of the second run.
2) with optcell 2 and ionmov 2,the full structure optimization is done.

Can anyone suggest what is the correct method for doing structural optimization?

Please shed some light on this.

Regards
Seba

optcell /= 0 is used for optimizing the unit cell of a solid. You have a molecule in a big box, you don't care about the box, you should use optcell 0.

Because you have only a molecule in a big empty box, you should also use only the gamma kpt, that is,
kptopt 0
nkpt 1
kpt 3*0.0

Sorry I was confusing with mentioning optcell. This is because I always put optcell=0 explicitly in my input file when I only move the ions.
But as this is the default value, you don't need it.

Jordan

Hai

Thank you for all the replies.

So as per your comments, I modified the input file for optimizing the atomic geometry of NO2 gas molecule as follows:
acell 10 10 10 (Bohr)
rprim 1 0 0
0 1 0
0 0 1
ntypat 2
znucl 7 8
natom 3
typat 2 1 2
xangst -1.094 0.463 0
0 0 0
1.094 0.463 0
ecut 10
kptopt 0
nkpt 1
nstep 10
diemac 2.0
ionmov 3
ntime 20
tolmxf 5.0d-5
toldff 5.0d-6
nband 10

Is the above input file could perform optimization of the geometry.

Please give some suggestions in this regard.

Thanks and Regards
Seba