Transient noise simulation does not converge

Hello,

I am running transient noise simulation on my circuit (Clock tree with square wave source)

i am using moderate errpreset, reltol, vabstol and iabstol and gmin are all the default values.

the transient noise simulation does not converge and I get the following errors: 

Notice from spectre at time = 1e-21 s during transient analysis `tran'.
Newton iteration fails to converge at time = 1e-21 s step = 1e-21 s.
Disaster recovery algorithm is enabled to search for a converged solution.
Error found by spectre at time = 160 ps during transient analysis `tran'.
ERROR (SPECTRE-16192): No convergence achieved with the minimum time step specified.

The following set of suggestions might help you avoid convergence difficulties.

1. Evaluate and resolve any notice, warning, or error messages.
2. Use realistic device models. Check all component parameters, particularly nonlinear device model parameters, to ensure that they are reasonable.
3. Small floating resistors connected to high impedance nodes can cause convergence difficulties. Avoid very small floating resistors, particularly small parasitic resistors in semiconductors. Instead, use voltage sources or iprobes to measure current.
4. Ensure that a complete set of parasitic capacitors is used on nonlinear devices to avoid jumps in the solution waveforms. On MOS models, specify nonzero source and drain areas.
5. Perform sanity check on the parameter values by using the parameter range checker (use ``+param param-limits-file'' as a command line argument) and heed any warnings. Print the minimum and maximum parameter value by using `info' analysis. Ensure that the bounds given for instance, model, output, temperature-dependent, and operating-point (if possible) parameters are reasonable.

6. Check the direction of both independent and dependent current sources. Convergence problems might result if current sources are connected such that they force current backward through diodes.

7. Enable diagnostic messages by setting option `diagnose=detailed'.
8. Use the `cmin' parameter to install a small capacitor from every node in the circuit to ground. This usually eliminates any jump in the solution.
9. Loosen tolerances, particularly absolute tolerances like `iabstol' (on options statement). If tolerances are set too tight, they might preclude convergence.
10. Try to simplify the nonlinear component models to avoid regions that might contribute to convergence problems in the model

Do you have any advice on how to achieve convergence? I do not want to use a liberal setting as that gives unreliable results for the amount of precision we need in capturing voltage noise values. 

I have tried different integration methods (gear2, trapgear, euler) as well as larger gmin but neither works.

thanks,

Haritha