N,N-Dimethylformamide: An interesting case

Here's N,N-Dimethylformamide in the gas phase compared to several ab initio computations using the structure found by Kimball.exe with standard parameters:
 
Dimethylformamide - Energies
---------------------------------------------------------------------------------------
                              Kimball     Gamess/6-31g  Gamess/6-31g,opt   G94W/6-31g,opt
Nucleus-Nucleus      Vnn   :  180.23058    180.23066       180.08970       180.08979
Electron-Electron    Vee   :  256.72510    264.77374       264.76824       264.76824
Nucleus-Electron     Vne   : -929.98466   -938.69044      -938.70460      -938.70460
Electronic energy    Eel   : -426.74952                                   -426.92614

potential energy     V     : -493.02898   -493.68604       -493.84666     -493.84667
kinetic   energy     T     :  246.51004    246.87115        247.01022      247.01022

total     energy     Etot  : -246.51894   -246.81489       -246.83644     -246.83644

Virial ratio        -V/T   :    2.000036     1.99977          1.99930        1.9993
Coordinates:
NC(CH3)              Å             :          1.4743             ¦             1.4612 (avg.)
NC(CHO)                    :          1.4743             ¦             1.4137
CO                         :          1.2102             ¦             1.2054
CH(CH3)                    :          1.1003             ¦             1.0843 (avg.)
CH(CHO)                    :          1.1003             ¦             1.0900
OCN                  deg   :        125.73               ¦           124.30
HCH(CH3)                   :        112.39               ¦           108.85
OCH(CHO)                   :        111.91               ¦           119.60
CNC(CH3,CH3)               :        106.36 (C's have     ¦           115.61
C(CH3)NC(CHO)              :        106.36  equal param.)¦           114.60
Dimethylformamide - Energies (Hartree)             This Trace file from Kimball.exe
-------------------------------------------------  comes from a second run of DMA
                                                   which created the figure at the top.
One electron energy               :  -684.81120    The numbers are slightly different
Two electron energy               :   257.85617    from the comparison table above.
Nuclear repulsion Energy          :   180.41847    In this run C4's distance from N1
                                    -----------    has been variied independently from
Total Energy                 Etot :  -246.53656    the N1C2 distance. Furthermore the
                                                   p-clouds were allowed to move along
Nucleus-Electron  pot.Energy Vne  :  -931.33467    the CO-bond. This created 22 variables
Electron-Electron pot.Energy Vee  :   257.85617    for the optimization and a slightly
Nucleus-Nucleus   pot.Energy Vnn  :   180.41847    more stable result. Several distances
                                    -----------    and angles have changed a bit. However
Total potential energy       V    :  -493.06003    the N1C4 bond has not moved near the
Total kinetic   energy       T    :   246.52348    much lower value of the ab initio 
Virial ratio                -V/T  :     2.00005    optimized structures. The reason is,
                                                   that Kimball presently cannot model well the
Electronic energy            Eel  :  -426.95503    partial NC4 double bond character of the
Electron potential           Vel  :  -673.47850    NC=O moiety. It gives, however, correctly
                                                   the planar arrangement of NCHO. The figure
DMA2 - Interatomic Distances and Angles            at bottom contains the numbered classes,            
                                                   referred to, and shows a better view on
NC(CH3)              :     1.4716526 Å             the NCHO region. The p-bond has moved
NC(CHO)              :     1.4699199 Å             nearer the C4 core and not, as perhaps
CH(CH3)              :     1.0987654 Å             expected, towards the more electronegative
CH(CHO)              :     1.0727132 Å             O core It appears that the  nearby N-atom
CO                   :     1.2184562 Å             and repulsion by the two compact oxygen 
OCN                  :   122.57056 deg             lone pairs make up for this result.
CNC(CH3)             :   106.71502 deg             At the next step an sp2 N-Atom with two
C(CHO)NC(CH3)        :   106.71502 deg             pz elctrons would share in a three-center
OCH                  :   113.50472 deg             4-electron p-bond system with C and O. This
NCH(CH3)             :   106.70849 deg             would generate a planar(C2)NCO moiety as
HCH                  :   112.08696 deg             obtained with most ab initio calculations, see
                                                   the discussion on p. 105/106 in "Exploring
DMA2 - Cartesian Coordinates                       Chemistry with Electronic Structure Methods",
                                                   2nd ed., J.B. Foresman & Ae. Frisch, Gaussian
12                                                 Inc. Pittsburgh, 1996.- Only the most exact
                                                   model chemistries with RHF-SCF give a nonplanar                                                 
N   0.             0.           0.                 geometry, as known to be true from experiment
C   0.42326436    -1.409471     0.                 Interestingly Sto-3G monitors a transition state
C  -1.4716526      0.           0.                 for the planar arrangement. Kimball is, of 
H   1.5220297     -1.4093458    0.                 course, too much nonplanar as expected from
H   1.0165492E-2  -1.8633589   -0.91138382         a pyramidal N-node with the lone pair at one
H   1.0165492E-2  -1.8633589    0.91138382         corner, see figure below.
H  -1.7875504      1.0523754    0.
H  -1.7875504     -0.52618769   0.91138382
H  -1.7875504     -0.52618769  -0.91138382
C   0.42276603     0.56837388  -1.2879769
O   1.1483951      1.543931    -1.3678916
H   0.12947952     0.17406669  -2.2415064
------------------------------------------------