[Pw_forum] Cohesive energy Potassium

[SAHIDA KURESHI]

Hello QE users,

I want to calculate binding energy of Potassium to graphene sheet. For that I am using following formula:
Ebe = {Etot - *(Nk * Ek - Nc*Ec)}/(Nk + Nc)
where Ebe = binding energy
      Etot = Total energy of system with K-adsorbed on Graphene sheet
      Nk = Number of Katoms
      Ek = Energy of single K atom 
      Nc = Number of carbon atom
      Ek = Energy of single C atom
For this calculation when I calculated energy of single K-atom in my super cell it comes out -1.0027 Ry However the Cohesive energy of K (experimental) is 0.93 eV/atom. I don't understand how should I get this value cohesive energy for K-atom. Here is my input file:


&CONTROL
calculation = 'scf',
nstep = 100,
restart_mode = 'from_scratch',
outdir = '/home/skureshi/BC/10/gga/BE/K'
pseudo_dir = '/home/skureshi/potential',
prefix = 'K',
lkpoint_dir = .false.,
verbosity = 'high',
wf_collect = .TRUE.,
etot_conv_thr = 5.0D-4,
forc_conv_thr = 2.0D-3,
/
&SYSTEM
ibrav = 14,
A = 7.5293928074005185, 
B = 7.5293928086089785,
C = 30.0,
cosAB = -0.49791288927551236,
cosAC = 0.0,
cosBC = 0.0,
nat = 1,
ntyp = 1,
ecutwfc = 60.0,
ecutrho = 240.0,
occupations = 'smearing',
degauss = 0.005,
vdw_corr = 'DFT-D',
/
&ELECTRONS
conv_thr = 1.0D-6,
electron_maxstep = 100,
mixing_mode = 'plain',
/
&IONS
ion_dynamics = 'bfgs',
ion_positions = 'from_input',
/
ATOMIC_SPECIES
K 39.100 K.pbe-mt_fhi.UPF
ATOMIC_POSITIONS {crystal}
K 0.5 0.5 0.5
K_POINTS {automatic}
 4 4 1  1 1 1                        
EOF

Thanking you,
Sahida 
Ph.D. student
School of Mechatronic Systems Engineering
Simon Fraser University
13450 102 Avenue, Surrey(BC)-V3T 0A3
CANADA