Other than a real circuit, SPICE deals with ideal components. In a completely symmetrical circuit this may lead to unexpected behavior. For example a simple astable multivibrator may not vibrate at all but assume a stable state. The circuit in figure 1 simulates a stable 39.7 mV on the out-pin without any tendency to oscillate at all.
Figure 1 symmetrical astable multivibrator is stable
You may force SPICE to assume a defined initial state for the simulation using the .IC directive (Initial Conditions, see „Adding LTSPICE directives“). In the above example, add
This puts the symmetrical circuit into an asymmetrical start condition and the multivibrator starts oscillating with a frequency of 7 kHz, as expected (figure 2).
Figure 2 oscillating output of the astable multivibrator
Alternatively, you could have clamped, for example, the base of one of the transistors to GND using
But heed this word of caution:
It may need some experimenting if your circuit proves stubborn. Remember to check the circuit for correctness before trying to solve simulation problems with .IC directives. Also, keep the number of .IC directives as small as possible to minimize unexpected side effects.
SPICE help topics to look at: .IC, Dot commands, Schematic editing
Harald Kapp, 2014-05-13