[jnimagine]

What is the geometry of dichloromethane?? Before optimization, Cl-C-Cl bond is 109.5, H-C-Cl bond is 67.8... so it must be tetrahedral... but after optimization, do the angles get bigger?? why?
and also, the bond length increases...
Why do angles and the bond length in dichloromethane increase when it's geometrically optimized??

Another question is...
when we examine (E)-1,2-diisopropylethene, (Z)-diisopropylethene, and 1,1-diisopropylethene... the molecular energies are different. (E) isomer has the least energy, so it's more stable and the 1,1-isomer's the least stable... i think...? why??? does it have to do with how the bulky groups are far apart or not?? 
The most confusing part!!! is how the bond angle for these isomers are in a weird order.... E has the smaller angle, and Z has the biggest.... I can't figure out why!!! > . <

[Borek]

Try to imagine sticking atoms of diferent sizes on 4 sides of carbon atom. Think in terms of repulsion between H&H, H&Cl and Cl&Cl atoms. When will the repulsion be large? Small? What will molecule do to accomodate?

[azmanam]

One of the problem with stick drawings of organic molecules is it has essentially no relationship to how big the atoms really are or how long the bonds really are.  Our drawings are so far from scale models it's crazy.  In reality, the bonds are much shorter than we draw them to be, and the atoms are much larger. 

For your second question, yes it absolutely has to do with how far apart the atoms are.  Here are what we call 'space filling models' of the molecules of diisopropylethylene.  You can see how close the atoms are. (For reference, in the static pictures I've colored the 2 carbon atoms of the alkene in red.  I hope it's clear).

(I tried including rotating gif's of the molecules, but the file size is a bit too large...  I hope these are clear enough...)