According to the section "Modifying CAMB" (https://cobaya.readthedocs.io/en/latest/theory_camb.html?#modifying-camb) in cobaya.readthedocs we can use CAMBParamRangeError, or simply CAMBError to assign a zero likelihood when the computation of any observable would fail. In my case the observable is omega_de and the fail is omega_de<0 and the MCMC chain stop when this occurs. Therefore, to use this Cobaya functionality, what I did was to put CAMBError in the function set_params in the python module results.py. The modified results.py is
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def set_params(self, params): """ Set parameters from params. Note that this does not recompute anything; you will need to call :meth:`calc_transfers` if you change any parameters affecting the background cosmology or the transfer function settings. :param params: a :class:`~.model.CAMBparams` instance """ self.Params = params if (self.omega_de < 0): raise CAMBError('omega dark energy is negative, assigning 0 likelihood')
Thank you very much.