ncpchem

The CHFClBr equilibrium geometry, optimised at the scalar relativistic CCSD(T) level (taken from here), was scaled in the range 0.5 - 2.0 and the total and E_PV energy was calculated at the B3LYP level. As pointed out by Peter the bond stretching will inevitably lead to a breakdown of the SCF model. Indeed one observes that the HOMO-LUMO gap closes and the SCF did not converge beyond scaling factor 1.4

The CCSD(T) optimized geometry is
r(C₁-Br): 1.976 Å
r(C₁-Cl): 1.776 Å
r(C₁-H): 1.100 Å

The nuclear repulsion energy is evidently not affected by the breakdown of the wavefunction model and drops off as 1/s, where s is the scaling factor.

The total energy can be fitted to a 5th-order polynomial within the range of validity of the model and has, as expected, a minimum near the scaling factor 1.0.

The total E_PV energy is not the most interesting quantity since the change in the chiral field for all atoms except the central carbon atom is rather complicated.

More interesting is the contribution to the E_PV energy from the central carbon atom. Surprisingly it increases within the range of validity of the model and goes through a minimum around s=0.82. It can be fitted well to a 4th-order polynomial.

    5
scf done: -3172.829144
 C    -1.874368     1.699460    -0.238135
Br    -2.517620    -0.079089     0.215619
 H    -2.245107     1.932961    -1.232587
Cl    -2.454927     2.930736     0.887073
 F    -0.528269     1.693590    -0.245441

r(C₁-Br): 1.945 Å
r(C₁-Cl): 1.766 Å
r(C₁-H): 1.087 Å
Total energy: -3206.489586 hartree
HOMO-LUMO gap: 0.248490 hartree
Total E_PV = 1.001E-18 hartree
E_PV (central carbon C₁): 5.523E-20 hartree

   13
scf done: -3477.261538
 C    -1.712432     1.322399    -0.468191  H  
 C    -2.083610    -0.032394    -0.003986  H  
 C    -2.498564     1.665678    -1.672890  H  
 C    -1.993900     2.307555     0.597662  H  
 C    -0.270315     1.354795    -0.798693  H  
 C    -3.139942     1.948107    -2.650403  H  
 C    -2.225567     3.109877     1.460851  H  
 C    -2.385463    -1.125021     0.370581  H  
 C     0.898869     1.383167    -1.067164  L H
Cl    -2.539540     4.202586     2.638111  H  
 H     1.935810     1.408304    -1.305458  H  
Br    -4.092424     2.366916    -4.103780  H  
 F    -2.707154    -2.290764     0.770012  L H

r(C₁-Br): 4.469 Å
r(C₁-Cl): 4.316 Å
r(C₁-H): 3.744 Å
Total energy: -3510.990470 hartree
HOMO-LUMO gap: 0.242571 hartree
Total E_PV = 1.629E-20 hartree
E_PV (central carbon C₁): 2.905E-23 hartree

   21
scf done: -3781.817398
 C    -1.569422     1.426972    -0.444525  H  
 C    -1.877703     0.020308    -0.116740  H  
 C    -2.369353     1.848010    -1.612073  H  
 C    -1.896220     2.286398     0.710845  H  
 C    -0.132460     1.555677    -0.761518  H  
 C    -3.025886     2.193654    -2.567233  H  
 C    -2.164591     2.990875     1.656343  H  
 C    -2.131541    -1.129727     0.151315  H  
 C     1.043019     1.661785    -1.021021  L H
 C    -2.466017     3.781276     2.718191  H  
 C     2.367315     1.781049    -1.313191  H  
 C    -3.763495     2.581033    -3.638934  H  
 C    -2.417583    -2.425916     0.453469  L H
 C    -4.422240     2.926515    -4.595711  H  
 C    -2.733502     4.485859     3.664755  H  
 C     3.542557     1.886953    -1.572586  H  
 C    -2.670204    -3.569979     0.720249  H  
 H     4.577709     1.980457    -1.801197  H  
Cl    -3.094602     5.433954     4.939276  H  
Br    -5.390750     3.433046    -5.999501  H  
 F    -2.937158    -4.778650     1.002047  H  

r(C₁-Br): 7.035 Å
r(C₁-Cl): 6.882 Å
r(C₁-H): 6.319 Å
Total energy: -3815.623118 hartree
HOMO-LUMO gap: 0.198085 hartree
Total E_PV = -9.436E-23 hartree
E_PV (central carbon C₁): -3.737E-25 hartree