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[taoj0003@ntu.edu.sg]

hi,guys

i m trying to optimize the lattice constant of rutile tio2. i m doing it in 
this way,
first, fix c=2.958 as experiment value, and then vary the 'a' from 4.55 to 
4.65 with a step 0.01,  the experiment value of 'a' is around 4.59. 
i am looking the Etotal energy, hope the energy reach a min value at around 
4.59. However, the result show the Etotal energy always decrease, meaning to 
take a optimized 'a' value,we need always inrease the value, which go very 
far from the experiment value. 
The ecut off energy i m using is 120 Hartree, should be quite good enough.

So any one can help me figure out the reason? thanks in advance.