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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