[Pw_forum] spin polarization of a organo-metallic compound

[Giuseppe Mattioli]

Dear Alex
Paolo is right, of course. Check the convergence on the Re PP first.
I would add a few words anyway. Hybrid DFT/HF functionals like B3LYP  
are pretty useful, but in the case of open-shell systems there are  
some known limitations. See the very interesting paper by Cohen  
(Science Vol. 321 no. 5890 pp. 792-794). The HF exchange term added to  
the DFT xc kernel leads to an overcoupling of electron pairs and,  
therefore, to an overestimate of the stability of low-spin solutions.  
Chemists usually call it a "static correlation" error. As we could  
spend years in discussing such an issue, I feel to suggest two simple  
tests: try to perform a B2PLYP calculation (if you can afford it) with  
your GTO package. The MP2 treatment of B3LYP correlation is supposed  
to correct part of the static correlation error. Otherwise, try to  
perform a DFT+U calculation with the U correction applied (possibly in  
a self-consistent fashion, see the many related papers by Cococcioni,  
De Gironcoli, Kulik, Marzari, and mine) to the Re 5d shell (I did  
never use Re... Are the 4f electrons embedded in the pseudo?). The  
on-site DFT+U method is supposed to be free from sce, or, better, less  
affected than its parent DFT functional. If you obtain a high-spin  
solution with one of these methods, there are sound reasons, in my  
opinion, to trust that it is a stable solution...
HTH
Giuseppe

Giuseppe Mattioli
ISM-CNR
Italy

Quoting Alex Granov <alex.granov at outlook.com>:

> Hi
> We are studying an organo-metallic compound which contains 3 Benzene  
> rings, phosphorous, oxygen, Iodine, and rhenium atoms.Our quantum  
> chemistry study using B3LYP/6-31+G* (Gaussian basis set) shows that  
> the ground state of the system is singlet (spin -unpolarized). We  
> attempted to optimize this structure using quantum espresso to  
> calculate some other electronic properties. Then surprisingly we  
> found that the spin-polarized system is more stable than the  
> spin-unpolarized one in contrast with our quantum chemistry study.  
> In the following you can see my input file and I wonder whether  
> there is something wrong in the input that made the very noticeable  
> contrast. I appreciate your comments to understand the issue and  
> solve it in advance.
> q/e input:&control    calculation='relax',     
> restart_mode='from_scratch',    prefix='Re-Be',    pseudo_dir =  
> './pseudo/',    outdir='./tmp/',    tprnfor=.t. ,     
> etot_conv_thr=1.0D-4,    forc_conv_thr=1.0D-3,    nstep=300,     
> dt=10,     / &system    ibrav= 8, a=20, b=20, c=20,  nat=37, ntyp=6,  
>    ecutwfc = 30, ecutrho = 300, occupations='smearing',  
> smearing='marzari-vanderbilt', degauss=0.005,    nspin=2 ,  
> starting_magnetization(1)=1 ,starting_magnetization(2)=1 ,   
> starting_magnetization(3)=1 , starting_magnetization(4)=1 ,  
> starting_magnetization(5)=1 , starting_magnetization(6)=1 , /  
> &electrons    conv_thr = 1.0d-6,    mixing_beta=0.2,     
> mixing_mode=local-TF, / &IONS  ion_dynamics="bfgs",  upscale=100.0D0  
> ,/ATOMIC_SPECIESRe 186.207  Re.pbe-hgh.UPFO  15.999    
> O.pbe-rrkjus.UPFC 12.000    C.pbe-rrkjus.UPFH 1.000      
> H.pbe-rrkjus.UPFI 126.90   I.pbe-n-rrkjus_psl.0.2.UPFP  30.9737   
> P.pbe-n-van.UPFK_POINTS {gamma}
>
> -------------------------------------------------Alex  
> GranovМосковский физико-технический институт (MIPT)Moscow, Russia


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    Giuseppe Mattioli
    CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
    v. Salaria Km 29,300 - C.P. 10
    I 00015 - Monterotondo Stazione (RM)
    Tel + 39 06 90672836 - Fax +39 06 90672316
    E-mail: <giuseppe.mattioli at ism.cnr.it>