[Wien] Negative strain energy due to isoelectronic substitution

[Stefaan Cottenier]

My first guess would be to suspect your constant RKMax value of 7. Your 
Rmt choices show that for BP it is the B-Rmt that is the smallest one 
(1.75), while for GaP it is the P-Rmt (1.85). If you use 
Rmt_min*Kmax=7.0 for both, your basis set for BP and GaP:B will be 
larger than for GaP. Adapting RKMax for GaP such that a larger basis set 
is used there as well, will lead you to ... an even more negative result.

Anybody comments on the above procedure?

Anyway, this alone isn't going to solve your problem. From the equation 
you give, it is not obvious to me that this should be positive. My naive 
expectation would be that if these two compounds like to mix, this 
equation could lead to a negative value. (?)

Stefaan

>Dear Wien2k users,
>
>would you please exchange your experience on calculation of the strain
>energy (sometimes also called 'formation enthalpy') of an alloy due to
>isoelectronic substitutional defects.
>
>I calculate a 64-atom GaP:B zinc-blend supercell. GaP:B means that one Ga
>atom is replaced my B atom. I let the supercell keep the equilibrium
>volume of GaP and relax internal degrees of freedom (atomic positions).
>Obtained structure I call "relaxed structure". I would like to figure out:
>What is the strain energy caused by substitutional B atom? For the stain
>energy I use the following definition [see e.g., Phys. Rev. B 63, 085207
>(2001)]:
>
>Estr = Etot(GaP:B) - (1-x)*Etot(GaP) - x*Etot(BP),
>
>where Etot(GaP:B) is the total energy of the relaxed structure, x is the
>concentration of substitutional atoms (in my case it is 1/32), Etot(GaP)
>and Etot(BP) are the total energies of 64-atom supercells of binary
>compounds at their equilibrium lattice constants.
>
>At the end I have:
>Etot(GaP:B) = -142238.0807 Ry
>Etot(GaP)   = -146071.2169 Ry
>Etot(BP)    =  -23402.7274 Ry
>which gives
>Estr = -0.254 Ry.
>
>This result surprises me due to its negative (!) sign. The strain energy
>should be at least positive. Note: The same approach works fine on PW
>codes (FHI98md). Maybe the problem is that the reference energy, with
>respect which the total energy is measured, in Wien2k changes itself from
>one structure to another? If such, would you please give me a suggestion
>how to overcome this problem.
>
>Thank you in advance.
>
>Yours,
>
>Oleg Rubel
>
>
>ADDITIONAL INFORMATION: Wien2k version: WIEN2k_06.2 (Release 9/03/2006).
>Run command ~>min -i 100 -s 10 -j 'run_lapw -p -I -i 40 -fc 1.0 ';
>minimization method: PORT; LDA; Spin polarization - NO; Rmt(Ga)=1.92 Bohr;
>Rmt(P)=1.85;  Rmt(B)=1.75; Kmax*Rmt_min = 7; 4x4x4 k-point mesh (shifted);
>Lmax = 10 (in *.in1c); for the rest parameters I rely on default values.
>
>
>===========================
>Dr. Oleg Rubel
>Faculty of Physics
>Philipps University Marburg
>Renthof 5, 35032 Marburg, Germany
>E-mail: Oleg.Rubel at physik.uni-marburg.de
>
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>  
>


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