[Pw_forum] Cell optimization converges to unrealistic value

[Yury Vishnevskiy]

Dear Giuseppe and all others,
first of all, sorry for the duplicated message in the list. I have not 
received any message to my initially registered e-mail address and was 
already thinking that my question has not got into the pw_forum.


 > Dear Yuri
 > There are technical and physical reasons behind your unrealistic results.

 > technical:
 >
 >>      N   14.00700  N.pbe-van_ak.UPF
 >>      H    1.00800  H.pbe-van_ak.UPF
 >>      C   12.01100  C.pbe-van_ak.UPF
 >>     Cu   63.54600  Cu.pbe-n-van_ak.UPF
 >
 >>      ecutwfc = 25 ,
 >
 >In the case of ultrasoft pseudopotential ecutrho has to be indicated, 
 >having a value between
 >6*ecutwfc and 10*ecutwfc (convergence should be reached with respect 
 >to all such values, but I
 >perform satisfactorily converged CuPc calculations with the same 
 >pseudos by using
 >ecutwfc=35/ecutrho=280 cutoffs...)


Ok, I will use no less then 35 and 280 for ecutwfc and ecutrho.


 >>      tot_charge = 0.000000,
 >>      starting_magnetization(1) = 0.0,
 >>      starting_magnetization(2) = 0.0,
 >>      starting_magnetization(3) = 0.0,
 >>      exxdiv_treatment = 'gygi-baldereschi' ,
 >
 >useless keywords (in this case...). Starting_magnetization(4) = x with 
 >0.0 < x < 1.0 is enough to
 >break the spin symmetry.
 >
 >>      nat = 33,
 >
 >The copper phthalocyanine (CuPc) has 57 atoms. Have you checked the 
 >structure?

I calculate a _polymer_ of CuPc. The unit cell I've created contains 33 
atoms, that should be correct.

 >>      ibrav = 6,
 >>      celldm(1) = 20.11677,
 >>      celldm(3) =  2.59367,
 >
 >your molecules embedded into a tetragonal cell seems to be placed at a 
 >z distance of about 30 A from
 >its periodically repeated image... Check the exact use of the 
 >celldm(i) variables...

Thank you! I was not careful in reading the manual and thought that 
these parameters are just lengths of vectors.

 >Physical:
 >
 >Molecular crystals stay together because of dispersion interactions. 
 >As you are not using any kind
 >of dispersion correction, you will never obtain meaningful results on 
 >lattice parameters. There is
 >plenty of theoretical papers investigating the CuPc molecule. I 
 >suggest to start with them...

Ok, thanks for pointing on that. Now I can play with calculations further.

Sincerely,
Yura