[Pw_forum] Bulk modulus of trigonal structures

[Nadire Nayir]

Dear all,
I deal with the cell optimization and the equation of states of the
trigonal structures (especially, GeO2). To get the optimized structure, I
started from the experimental structure and performed a series of the relax
calculations with "relax" keyword to it. During the relax calculations,
first, I changed celldm(1)=a and celldm(3)=c/a parameters by keeping c
parameter constant and I found the optimized celldm(1), after that, I kept
celldm(1) fixed at its optimized value and I started to change only
celldm(3) parameter until getting the optimized c parameter.

After getting the optimized celldm(1) and celldm(3), I calculated the
equation of states of the structure by applying SCF calculation and
finally, I used ev.x to calculate the bulk modulus with the non-cubic
option, which is ~140 GPa.

However, the bulk modulus value I found is so different from the
experimental value which is 42 GPa.

When I applied the same optimization process to the cubic structures
(Si-diamond and Ge-diamond), I achieved the almost same results with their
experimental values.

I did not understand why this procedure does not work for the trigonal
structures. I wonder, is there anything that I missed?

By the way, I also applied vc-relax calculation to the trigonal structure,
again, I got the larger bulk modulus than the experimental one!

I would greatly appreciate if you could kindly help me with this problem.

Best regards,
Nadire


-- 
Nadire Nayir
Research Assistant
Middle East Technical University
Physics Department
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