[Pw_forum] Modelling the pyramidization of Disilane using QE

[David Bruce Williams-Young]

Hey all,

I'm relatively new to using QE, and I've run into a bit of a problem.

I'm attempting to model the pyramidization of Disilane from planar 
geometry. Disilane (Si2H4) has the same structure as ethene (H2C=CH2), 
but where ethene is planar, disilane is bent from the planar. This 
distortion is due to the fact that in ethene, the CH2 fragments favour 
the triplet spin state (multiplicity = 3) and the Disilane, the SiH2 
fragments favour the singlet spin state (multiplicity = 1). The 
bonding of two singlet fragments causes a distortion of the SiH2 
planes from the plane of the two Si's (one up and one down).

So my question is, does anyone have any idea how I can model this 
phenomena using the 'relax' calculation (if there's a better one, 
please let me know).

Also, I've been able to, on other computational chemistry software, to 
calculate the total energy of the fragments (:CH2 vs :SiH2), but I'm 
trying to use QE to do it now (just to have consistency of software). 
I attempted to run a 'relax' calculation of a bent CH2 fragment (made 
in ECCE), using

&System
  ...
  multiplicity = 1

and

&System
  ...
  multiplicity = 3

I can't seem to get the calculation to work properly, as whenever I 
run pw.x with the multiplicity specified, I only get '2' echoed in 
terminal. How might I run this calculation?

I'd be more than happy to clarify on any part of this!

Thank You,
David