BSSE calculations

[Wheeler, Dr M.D.]

Dear All,
I am currently trying to map out the PES of various Rg-CC complexes,
however, I am running in to a few problems.  When I do the calculations
using a do loop in the input file there appears to be a problem close to
linear geometries.  Depending upon which direction I scan the PES I get
very different answers (a minimum if I increase theta and a maximum if I
decrease theta), I use the following input file,


 ***, CC-Rg intermolecular PES, CCSD(T), aug-cc-pVDZ basis
 Memory,80,m;
 basis=avtz                                     !basis set
                                                !****define internal
coordinates for complex****
 rcc=1.25                                       !CC bond-length
 rc=rcc*0.5                                     !distance of C from
center of mass, RC
 distances=[3.825]
                                                !distances of Rg from
center of mass, R_Rg
 angles=[170,175,180]
                                                !Rg-CoM-C angle, theta
 geometry={x,angstrom;                          !****define RgCC complex
using two dummy atoms****
 q1;
 q2,q1,5.0;
 c1,q1,rc,q2,90.0;
 c2,q1,rc,q2,270.0,c1,0.0;                      !geometry of CC
 ne,q1,r_rg(i),q2,90.0,c1,theta(i)              !geometry of Rg
 }
                                                !****start loop over
variables****
 i=0
 do ith=1,#angles
 do ir=1,#distances
 i=i+1
 r_rg(i)=distances(ir)
 theta(i)=angles(ith)
                                                !****start actual
calculation here****
 text,calculation for complex
 dummy
 hf;                                            !SCF for total system
 e_complex_hf(i)=energy                         !save energy in variable
e_complex_hf(i)
 ccsd(t);                                       !CCSD(T) for total
system
 e_complex(i)=energy                            !save energy in variable
e_complex(i)
  
 text,cp calculation for CC
 dummy,ne                                       !make Rg a dummy center
 hf;                                            !scf for CC
 e_cc_hf(i)=energy                              !save energy in variable
e_cc_hf(i)
 ccsd(t);                                       !CCSD(T) for CC
 e_cc(i)=energy                                 !save energy in variable
e_cc(i)
  
 text,cp calculation for Rg
 dummy,c1,c2                                    !make CC dummy
 hf;                                            !scf for Rg
 e_rg_hf(i)=energy                              !save energy in variable
e_rg_hf(i)
 ccsd(t);                                       !CCSD(T) for Ne
 e_rg(i)=energy                                 !save energy in variable
e_rg(i)

 de_cp_hf(i)=(e_complex_hf(i)-e_cc_hf(i)-e_rg_hf(i))*tocm
 de_cp(i)=(e_complex(i)-e_cc(i)-e_rg(i))*tocm
 enddo
 enddo

                                                !****exit loop and
create table of results****
!table,theta,r_rg,e_complex_hf,e_cc_hf,e_rg_hf,de_cp_hf,e_complex,e_cc,e
_rg,de_cp
table,theta,r_rg,de_cp
! head,
theta(Deg),R_Rg(ang),E_complex_hf,E_CC_hf,E_Rg_hf,De(CP_cor)_hf,\
! E_complex,E_CC,E_Rg,De(CP_cor)
 save,necc_bend_test_x.tab
! title,Results for Ne-CC (energies in cm-1), basis $basis
 ---;

Can anyone suggest why this might be happening...???

Martyn


----------------------------------------------
Dr. Martyn D. Wheeler
Department of Chemistry
University of Leicester
University Road
Leicester, LE1 7RH, UK.
Tel (office): +44 (0)116 252 3985
Tel (lab):    +44 (0)116 252 2115
Fax:          +44 (0)116 252 3789
Email:        martyn.wheeler at le.ac.uk
http://www.le.ac.uk/chemistry/staff/mdw10.html