Dear Siva,
The computation of the exchange integrals is very costly, and, naively, scales as O(N^4) with the basis set size N. Hence, the ADMM method introduces an auxiliary, smaller basis to compute the exchange integrals. What this means in practice is that you can use large, high quality basis sets (like the MOLOPT basis sets mentioned in your question), and then specify a smaller auxiliary basis for your elements to enable the ADMM method, and speed up the HF calculations.
Apart from including the &AUXILIARY_DENSITY_MATRIX_METHOD section in the appropriate place, you need to choose which auxiliary basis sets you want to use for each element.
the ADMM basis sets are explained, and the input file structure is given as well.
For your convenience, I attach an input file for TiO2 using PBE0 and the ADMM method, as well as the basis files BASIS_ADMM and BASIS_ADMM_MOLOPT, which contain the auxiliary basis sets.
As a side note, if you are using an older version of CP2K, you may need to specify the auxiliary basis sets via:
AUX_FIT_BASIS_SET ...
instead of:
BASIS_SET AUX_FIT ...
in the &KIND section of your input.
Yours sincerely,
Patrick Gono