HSPCIE Default Value for Float Nodes
Hspice will generate an error:


  Node 0 is always ground and floating nodes (a node from which there is
  no connection to ground through circuit elements) will generate an
  error.


Source: MIT Spice Tutorial 

The solving matrix can't solve for an equation with a floating node. If you really need to simulate what this would be physically attach a really large resistor on the order of \$ 10^9\$ ohms (or the resistance of air or FR4) to your 'floating node'. Or you could couple them capacitively with a large resistor, physically there are not many situations where you would get a 'floating node' as air and other materials have resistance that need to be accounted for when dealing with very small currents.