# Cohesive energy calculated by ReaxFF for fcc Gold much higher than EAM force field

Dear all,

I am trying to test the ReaxFF from bulk optimization for fcc Au lattice. By changing gold lattice parameter from 3.0 to 5.0 Angstrom, I got the right Energy VS distance curve shape as below. However, the total energy and pressure calculated are extremely high about -333 eV and 5505330 bar when lattice constant is 4.0 Angstrom. But for EAM potential, the energy and pressure is good - -15.58eV and 115574 bar.

The optimization curve for ReaxFF is attached. The input file of reaxFF is below, which I revised some parts from the lammmps tutorial 2 of Al to reaxFF of Au.

Do I need to add another fix command to fix temperature ? What is the key reason that the total energy and cohesive energy calculated by this script are so high?

6 clear

7 units metal

8 dimension 3

9 boundary p p p

10 atom_style charge

11

12

13 # ---------- Create Atoms ---------------------

14 lattice fcc \${latconst}

15 region box block 0 1 0 1 0 1 units lattice

16 create_box 1 box

17

18 lattice fcc \${latconst} orient x 1 0 0 orient y 0 1 0 orient z 0 0 1

19 create_atoms 1 box

20 replicate 1 1 1

21

22 mass 1 196.966

23 set type 1 charge 0.0

24

25 # ---------- Define Interatomic Potential ---------------------

26 pair_style reax/c NULL

27 pair_coeff * * ffield.reax.AuO Au

28

29

30

31

32 # ---------- Run 0 ---------------------

33 fix 1 all qeq/reax 1 0.0 10.0 1e-6 reax/c

34 run 0

35

36 variable natoms equal “count(all)”

37 variable teng equal “pe”

38 variable length equal “lx”

39 variable ecoh equal “v_teng/v_natoms”

40

41 print “Total energy (eV) = \${teng};”

42 print “Number of atoms = \${natoms};”

43 print “Lattice constant (Angstoms) = \${length};”

44 print “Cohesive energy (eV) = \${ecoh};”

45 print “%% ecoh = \${ecoh};”

46

47 print “All done!”

Zhi

Dear all,

I am trying to test the ReaxFF from bulk optimization for fcc Au lattice. By
changing gold lattice parameter from 3.0 to 5.0 Angstrom, I got the right
Energy VS distance curve shape as below. However, the total energy and
pressure calculated are extremely high about -333 eV and 5505330 bar when
lattice constant is 4.0 Angstrom. But for EAM potential, the energy and
pressure is good - -15.58eV and 115574 bar.

The optimization curve for ReaxFF is attached. The input file of reaxFF is
below, which I revised some parts from the lammmps tutorial 2 of Al to
reaxFF of Au.

Do I need to add another fix command to fix temperature ? What is the key
reason that the total energy and cohesive energy calculated by this script
are so high?

6 clear

7 units metal

wrong units?

axel.

In addition, there is no meaning in comparing the absolute energies from different potentials at specific lattice constants. What makes sense is to look at the shape and 1st derivative of the E-V curve from each of the different potentials.

Ray