[Pw_forum] relaxation of even-layer slab

[lucky8121]

Dear users,

I am a new user of PWSCF. I am doing the structure relaxation of metal slabs.
The relaxations (interlayer spacings) on the two sides of the odd-layer slab are symmetric. However, I met problem with the relaxation of even-layer slabs.

The initial positions of the Cu(111) slab were:
Cu       0.000000000   0.577350269190  -1.224744871392
Cu       0.000000000   0.000000000000  -0.408248290464
Cu       0.500000000   0.288675134595   0.408248290464
Cu       0.000000000   0.577350269190   1.224744871392

After relaxation (forc_conv_thr = 4.0D-4), the final positions are:
Cu       0.000000000   0.577350269  -1.207031882
Cu       0.000000000   0.000000000  -0.404842681
Cu       0.500000000   0.288675135   0.404671325
Cu       0.000000000   0.577350269   1.207203237

As you can see, the relaxations on the two sides of the slab are different. The same case occured whether I set "nosym" as ".true." or ".false.". This is strange. I wonder whether someone else has met the same problem and how he or she solved it. Thanks.

By the way, I have used a vacuum of 12.7 angstrom.

Best regards

lucky
lucky8121 at sohu.com
2006-10-11