Distance between pull groups 1 and 2 (2.877739 nm) is larger than 0.49 times

the box size (2.272821).

But my box dimension are 4.69, 4.69, 15 . Here I am pulling the drug on Z direction why does it taking directions of x and y as i am not doing anything on that direction. the smd code is as follows

pull = yes

pull_ncoords = 1 ; only one reaction coordinate

pull_ngroups = 2 ; two groups defining one reaction coordinate

pull_group2_name = DRUG

pull_group1_name = Lipid

pull-group2-pbcatom = 34491

pull-group1-pbcatom = 3903

pull_type = umbrella ; harmonic potential

pull_geometry = direction ; simple distance increase

pull_dim = N N Y

;pull_coord1_vec = 0 0 -1

pull-pbc-ref-prev-step-com = yes

pull_coord1_groups = 1 2

pull_coord1_start = yes ; define initial COM distance > 0

pull_coord1_rate = 0.00025 ; 0.01 nm per ps = 10 nm per ns

pull_coord1_k = 100 ; kJ mol^-1 nm^-2

pull_init1 = 0