[Pw_forum] Re: Why the energies are different when two molecules are calculated together and separate?

[Xunlei Ding]

Dear All,

Thank you  for all the useful discussion.

I have tested the cutoff both for the wavefunction and ecutoff. The 
results is below.
The first line indecate the cutoff, such as 24Ry288 means ecutwfc = 
24.0, ecutrho = 288.0.
The 2-5 lines are the total energies of HCO2 (in one supercell), CO2, H, 
CO2+H (just the sum of line3 and line4).
And the 6th line is the energies between E(HCO2) and E(H)+E(CO2) in eV.

                            24Ry288    32Ry400     32Ry600    40Ry600
HCO2 (Ry)         -76.156        -76.202        -76.202        -76.205
CO2 (Ry)            -75.144        -75.188        -75.188        -75.190
H atom (Ry)          -0.998          -0.999          -0.999          -0.999
CO2+H (Ry)       -76.142        -76.187        -76.187       -76.189
delta (eV)              -0.187          -0.206          -0.205         
-0.212

It can been seen that the cutoff will influence the energy difference 
little in this case.

I also think the charge transfer may pay an inportant role for this 
problem.  So I have done projwfc.x calculation, but there is error in 
the output file,
like:
     e =   -29.29871 eV
     psi =   Inf*[#  2]+  Inf*[#  6]+  Inf*[#  8]+  Inf*[# 10]+  Inf*[# 12]+
          +******[#  1]+******[#  9]+******[# 13]+******[# 11]+******[#  7]+
          +******[#  5]+******[#  4]+******[#  3]+
The expansions for alpha wavefunction are all with Inf coefficients. So 
the charges are not provided by this calculation.

So should I calculate the differential charge density to see the charge 
transfer?

I used spin=2 calculation for HCO2 and H atom, but spin=1 for CO2.  It 
has been tested that spin=2 for CO2 will give almost the same results.

I think the input files for these calculations are worth to be checked. 
I list them below.

Thank you all!

Yours sincerely,
Ding

------------------------------For HCO2:---relax is converged in one 
step------------------------------
&control
    calculation='relax',
    restart_mode='from_scratch',
    pseudo_dir = '/scratch/cne0fm32/dxl/pseudo/',
    outdir='/scratch/cne0fm32/dxl/test' , prefix='HCO2',
    tprnfor = .true.,
    tstress = .true.
 /
 &system
    ibrav=8, celldm(1) =28.345891875, celldm(2)=1.0, celldm(3)=1.0, 
nat=4, ntyp=3,
    nspin = 2,  starting_magnetization(1)=1.0,
    ecutwfc = 24.0, ecutrho = 288.0,
    occupations='fixed'
    nelup=9, neldw=8, nelec=17
 /
 &electrons
    electron_maxstep= 200
    diagonalization=''
    conv_thr = 1.0e-6
    mixing_beta = 0.2
 /
 &ions
  ion_dynamics = 'bfgs'
 /
ATOMIC_SPECIES
 H  2.00  H.pbe-rrkjus.UPF
 C 12.00  C.pbe-rrkjus.UPF
 O 16.00  O.pbe-rrkjus.UPF
ATOMIC_POSITIONS
H        0.0000        0.4  0.00
C        0.0000        0.000000000  0.00000000
O        0.078114541   0.000000000  0.00000000
O       -0.078114541   0.000000000  0.00000000

---------------------------for CO2-----relax is converged in one 
step---------------------------
&control
    calculation='relax',
    restart_mode='from_scratch',
    pseudo_dir = '/scratch/cne0fm32/dxl/pseudo/',
    outdir='/scratch/cne0fm32/dxl/test' , prefix='CO2',
    tprnfor = .true.,
    tstress = .true.
 /
 &system
    ibrav=8, celldm(1) =28.345891875, celldm(2)=1.0, celldm(3)=1.0, 
nat=3, ntyp=2,
    nspin = 1,  starting_magnetization(1)=0.0,
    ecutwfc = 24.0, ecutrho = 288.0,
    occupations='fixed'
 /
 &electrons
    electron_maxstep= 200
    diagonalization=''
    conv_thr = 1.0e-6
    mixing_beta = 0.2

 /

 &ions
  ion_dynamics = 'bfgs'
 /

ATOMIC_SPECIES
 C 12.00  C.pbe-rrkjus.UPF
 O 16.00  O.pbe-rrkjus.UPF

ATOMIC_POSITIONS
C        0.0000        0.000000000  0.00000000
O        0.078114541   0.000000000  0.00000000
O       -0.078114541   0.000000000  0.00000000

K_POINTS (gamma)

---------------------------For H atom--------------
&control
    calculation='scf',
    restart_mode='from_scratch',
    pseudo_dir = '/sfs/sanfs/home/userinfm/cne0fm32/dxl/pseudo',
    outdir='./' , prefix='H',
    tprnfor = .true.,
    tstress = .true.
 /
 &system
    ibrav=8, celldm(1) =28.345891875, celldm(2)=1.0, celldm(3)=1.0, 
nat=1, ntyp=1,
    nspin = 2, starting_magnetization=1.0
    ecutwfc = 24.0, ecutrho = 288.0,
    occupations='fixed'
    nelup=1, neldw=0, nelec=1

 /
 &electrons
    electron_maxstep= 200
    diagonalization=''
    conv_thr = 1.0e-6
    mixing_beta = 0.2

 /

 &ions
  ion_dynamics = 'bfgs'
 /

ATOMIC_SPECIES
 H 2.00  H.pbe-rrkjus.UPF

ATOMIC_POSITIONS
H    0.0   0.000000000  0.00000000

K_POINTS (gamma)