I suffered a problem which involves decribing the monoclinic crystal system in the input file. This problem is easy to solve in other software like VASP or QE, but I didn't find a way to do that in exciting.
could you, please, specify more exactly on what is the problem?
Generally speaking, I don't think that describing lattice structures in VASP is that different from exciting. First, you need to know what are the lattice vectors that define your structure. Second, you have to provide positions of atoms in lattice/fractional or Cartesian coordinates. Alternatively, you can provide them in Cartesian coordinates. In any case, you have to repeat these two steps for any kind of crystals regardless whether you use VASP or exciting.
Thanks for your response. Here is my system TiO2 (B-type)
Space group is C 2/m, monoclinic
a = 12.1787A, b=3.7412A, c=6.5249A, beta = 107.054
The coordinates are
Ti (0.1970,0.0000,0.2920) and (0.0990,0.0000,0.7050)
O (0.1320,0.0000,0.0040), (0.2640,0.0000,0.6530), (0.0600,0.0000,0.3700)
How can I describe them in the input file? It seemly takes quit a complicated calculation.
If you have managed to feed your structure into VASP and have a POSCAR file already, you are one very small step away from having the input for excitng. But I assume that you are starting from scratch and will describe how would I have prepared the input step by step.
If you do not have VESTA on your computer, download it. VESTA is a visualisation tool with a support of many formats and has many useful features. If you have a CIF file with the structure, just read it in, and use VESTA to export data to the POSCAR format (File -> Export Data…).
If you have only the data that you have written above, follow the steps below. These steps also require VESTA. They are a bit tedious, but very straightforward.
- Create a new structure (File -> New).
- Choose the space group and enter the lattice parameters (Edit -> Edit Data… -> Unit cell).
- Specify positions of all atoms (Edit -> Edit data… -> Structure parameters).
- If you wish you can visualise bonds to verify that the structure is right (Edit -> Bonds…).
- Export data to the POSCAR format and choose "Fractional coordinates", when asked.
After these steps I get a POSCAR file with the following content.
New structure 1.0 12.1787004471 0.0000000000 0.0000000000 0.0000000000 3.7411999702 0.0000000000 -1.9135762354 0.0000000000 6.2379921027 Ti O 8 16 Direct 0.098999999 0.000000000 0.704999983 0.901000023 0.000000000 0.295000017 0.598999977 0.500000000 0.704999983 0.400999993 0.500000000 0.295000017 0.196999997 0.000000000 0.291999996 0.802999973 0.000000000 0.708000004 0.697000027 0.500000000 0.291999996 0.303000003 0.500000000 0.708000004 0.131999999 0.000000000 0.004000000 0.868000031 0.000000000 0.995999992 0.631999969 0.500000000 0.004000000 0.368000001 0.500000000 0.995999992 0.263999999 0.000000000 0.652999997 0.736000001 0.000000000 0.347000003 0.763999999 0.500000000 0.652999997 0.236000001 0.500000000 0.347000003 0.361999989 0.000000000 0.293000013 0.638000011 0.000000000 0.707000017 0.861999989 0.500000000 0.293000013 0.138000011 0.500000000 0.707000017 0.059999999 0.000000000 0.370000005 0.939999998 0.000000000 0.629999995 0.560000002 0.500000000 0.370000005 0.439999998 0.500000000 0.629999995
Then, you have to convert it to exciting xml format. The lattice is defined as follows. The scale in this case represents the factor of conversion between Å and bohrs.
<crystal scale="1.889725989"> <basevect>12.1787004471 0.0000000000 0.0000000000</basevect> <basevect> 0.0000000000 3.7411999702 0.0000000000</basevect> <basevect>-1.9135762354 0.0000000000 6.2379921027</basevect> </crystal>
The positions of atoms can be used directly as they are in POSCAR. Personally I would have used a spreadsheet program to generate this part of th input.
<species speciesfile="Ti.xml"> <atom coord="0.098999999 0.000000000 0.704999983" /> ... </species> <species speciesfile="O.xml"> <atom coord="0.131999999 0.000000000 0.004000000" /> ... </species>
By this point, the geometry part of the input is generated. There could be other ways of doing the same, so it is up to you choose how exactly you would like to do it.
Unbelievable, it works! I really appreciate you have offered a patient and careful step-by-step guide to me as every step can puzzle the novice for a long time. Right now I can convert any systems by using this method.
what is the significance of negative parameter in the lattice vector description any one will u explain please !