The voltmeter is modelled as a large resistance which does not disturb the current in the remaining circuit. The problem is that the branch containing the voltmeter is a stiff branch that appears in the loop equations. These equations appear randomly. So in the above circuit, when the simulation begins, suppose diodes D1 and D6 conduct. This means voltage vac appears across the capacitor Cdc and the voltmeter Vo. What would need to happen is that the voltmeter loop should be written with Cdc and therefore as Cdc charges, Vo output voltage should increase. But what is Vo appears in the loop with vac, D1 and D6? This is perfectly possible the way the program works now. This means Vo will jump to the voltage vac even though Cdc is charging gradually.
So Vo does not reflect the voltage that appears across the nodes but rather on the voltage impressed on it through the loop it appears on. And this inevitably happens whenever the circuit becomes large and loop equations become more and more random. Also, if the voltmeter were connected across a largely inductive circuit or for that matter over two fairly distant nodes in the circuit. What then?
The only error free way to solve this seems to be to do a nodal analysis with only the voltmeters for the nodes where they exist. This calls for a fairly large additional piece of code that will run at simulation time step.