[lammps-users] A query about input script

Hi all,
       I am new to LAMMPS and i am trying to perform to biaxial molecules on higher density.
In the output listed while perform, i found that the pressure is having a negative value.
I tried to perform only NVT simulations the pressure term displays a negative value.
Could you please kindly let me know why is it. Herewith i have attached my input script.
Kindly let me know what can i do.

variable T        equal 2.8
variable P        equal 1.20

variable rho      equal 0.30

# ------------------------------------------------------------------------------
# ----------- GayBerne ellipsoids in a 3D  -------------------------------
# ------------------------------------------------------------------------------
units             lj
atom_style        ellipsoid
dimension         3
boundary          p p p

lattice            sc ${rho}
region             box block 0 10 0 10 0 10
create_box        1 box
create_atoms      1 box

set               type 1 mass 1.0
set               type 1 shape 1.4 0.714 3.0
#set               group all quat/random 123456782

set               type 1 quat 0.892 0.09 0.095 0.367

variable gb_ps_gamma    equal 1.0
variable gb_ps_upsilon  equal 3.0
variable gb_ps_mu       equal 1.0
variable gb_ps_cutoff   equal 4.0

pair_style  gayberne    ${gb_ps_gamma} ${gb_ps_upsilon} ${gb_ps_mu} ${gb_ps_cutoff}

variable gb_pc_epsilon  equal 1.0
variable gb_pc_sigma    equal 0.714

variable gb_pc_epsilon_i_a  equal 1.7
variable gb_pc_epsilon_i_b  equal 1.2
variable gb_pc_epsilon_i_c  equal 0.2

variable gb_pc_epsilon_j_a  equal 1.7
variable gb_pc_epsilon_j_b  equal 1.2
variable gb_pc_epsilon_j_c  equal 0.2
variable gb_pc_cutoff       equal 4.0

pair_coeff  * * ${gb_pc_epsilon} ${gb_pc_sigma} &
    ${gb_pc_epsilon_i_a} ${gb_pc_epsilon_i_b} ${gb_pc_epsilon_i_c} &
    ${gb_pc_epsilon_j_a} ${gb_pc_epsilon_j_b} ${gb_pc_epsilon_j_c} ${gb_pc_cutoff}

two comments:

  1. your input file is a textbook example for how to not use variables. the primary effect you achieve is making your input much harder to read.
  2. a negative pressure means that the system “wants to shrink”.