# How to calculate lattice parameter?

After 'vc-relax' calculation I got the cell parameters in the picture bellow. I want to mention that my ibrav=1.

I think the optimized lattice parameter in this case is A=11.44879234*1.006676454 .But I'm not sure because I don't know if I have to include the number 0.000089189 . Can someone correct me please? • Camila, the one-topic-per-answer tag is used for question like this: mattermodeling.stackexchange.com/q/392/5 in which each answer attempts to explain one topic (in that case, "double-hybrid functionals" was one topic, "dispersion corrections" was another topic, "density-corrected DFT" was another topic, etc.). Thanks for re-tagging @IanBush ! Mar 3 at 23:56

In your output file, there should be a line at the beginning saying "crystal axes: cart. coord. in units of alat)" like the following:

     crystal axes: (cart. coord. in units of alat)
a(1) = (   1.000000   0.000000   0.000000 )
a(2) = (   0.000000   1.000000   0.000000 )
a(3) = (   0.000000   0.000000   1.000000 )


Check these three lines to obtain the lattice vector. For example, if it is like the above, then what you have done is correct. If it is, let's say 0.5 instead of 1, then you have to divide what you have got with 0.5.

You don't have to consider other numbers to get the lattice parameter since for ibrav=1, the lattice vector components are 0. But it is always a good practice to check the output file to make sure your lattice vectors are as you would expect. Also note that the unit is in Bohr.

• Thank you. So for the CELL_PARAMETERS matrix we consider just the element of the diagonal. Is it the case for other ibrav values? Mar 5 at 21:24
• No, this is not necessarily the case for other ibrav values. Here you will find the primitive lattice vectors for different ibrav values. When the primitive lattice vectors coincide with the cartesian basis vectors, we can only consider the diagonal elements. In general, one should consider all the values. Note that in the CELL_PARAMETER card, the three lattice vectors are expressed in cartesian coordinates. Mar 5 at 21:37
• This conversation is also happening here and here Mar 7 at 6:48