[Pw_forum] Cohesive energy Potassium

[Giovanni Cantele]

two remarks:

1) the total energy coming from a single scf calculation cannot be the cohesive energy you are looking for. For any code based on plane waves, the total energy is meaningless, because the zero energy is undefined. As a side effect, if you change the pseudo potential, you get a different total energy of the same system. The cohesive energy of K you are looking for is probably the cohesive energy of a K crystal. So, you need two calculations, one for the K crystal, with its native unit cell, one for an isolated K atom (spin polarised run if you expect a magnetic ground state). If the K crystal unit cell has l_K atoms, than the cohesive energy is defined as
(E_K_crystal - l_K * E_K_atom) / l_K

2) if I’m not wrong, the formula you give is kind of “formation energy” of a graphene sheet in the presence of an adsorbed K atom (it contains a wrong sign), that is, the energy you require to bring Nk K atoms and Nc C atoms far from each other to the configuration of the K-adsorbed graphene sheet. However, the binding energy is usually defined as the energy required to adsorb the K atom to an already formed graphene sheet, that is
E_binding_K = Etot_K+graphene - E_graphene - E_K

Giovanni


> On 21 Oct 2015, at 10:04, SAHIDA KURESHI <skureshi at sfu.ca> wrote:
> 
> 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
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