You want to chant the mantra in complete inner silence, mānasika-japa, with the mind alone? You're millions of years too late — that was the yoga-dharma of Satya-yuga. Now it's Kali-yuga, and the method is different: chant aloud. Let both work together — rasanā, the tongue, and mānasī, the mind. Silent japa in the mind alone is already smaraṇa, remembrance, not kīrtana. And the mission of Sri Chaitanya Mahaprabhu is precisely kīrtana, the Name spoken loud.