I'm looking to implement a custom dark energy model in CAMB. How do I define a new dark energy class, and what are the specific variable names or indices for extracting the cross-correlation power spectrum between the Cold Dark Matter (CDM) density and the Weyl potential?
Search for the official CAMB documentation online. Navigate to the 'Modifying the code' section of the CAMB documentation. Identify the general requirements for defining new cosmological component classes, specifically for dark energy. Locate the 'Matter power spectrum and matter transfer function variables' section. Find the variable name and numerical index for CDM density. Find the variable name and numerical index for the Weyl potential. Synthesize the information on defining a new dark energy class and the specific variable names for the desired cross-correlation.
State that defining a new dark energy class requires modifying both Python and Fortran code. Mention the use of the `@fortran_class` decorator in Python and the significance of `_fields_` for mapping Fortran variables. Identify 'delta_cdm' (or its numerical index 2) as the variable name for CDM density. Identify 'Weyl' (or its numerical index 10) as the variable name for the Weyl potential. Confirm that the cross-correlation power spectrum can be obtained by requesting these two variables (e.g., var1='delta_cdm', var2='Weyl').