lattice vectors change wih ISIF=2

[jk2]

Hi,
I am performing structural relaxation (ISIF=2). My initial lattice vectors are:

7.720000 0.00000 0.00000
0.000000 7.72000 0.00000
0.000000 0.00000 27.452320

and in CONTCAR file I see :

7.7199999999999998 0.0000000000000000 0.0000000000000000
0.0000000000000000 7.7199999999999998 0.0000000000000000
0.0000000000000000 0.0000000000000000 27.4523200000000003

Of course, the difference is small, but why it happened?

thanks,
jan

[tjf]

Presumably 7.72 is not representable in the representation model used on your machine (something like a 52 binary bit mantissa). What you get is the nearest possible number you can store on your machine.

To learn more, look up binary floating point data structures. (Googling 'ieee real' will give you a heap of info.)