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

Xunlei Ding ding at sissa.it
Thu Apr 13 10:24:26 CEST 2006

```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)

```

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