The best mantra for Atma Shanti as per Hinduism is being content and to stop searching for happiness in external things. Below is a mantra for atma shanti and it should be chanted sitting facing east or northeast. It is also good to chant the mantra during sunrise.
Mantra For Atma Shanti
श्वेते वृषे समारूढा,श्वेताम्बरधरा शुचि:।
महागौरी शुभं दद्यात्,
महादेवप्रमोददाद।।
Shweta Vrishabe Samaruda Shwetambaradhara Shuchi
Mahagauri Shubhamddyat Mahadevpramoddath
The meaning of the Mantra in Hindi
सफेद वृषभ (बैल) पर सवारने वाली, श्वेत वस्त्र धारण करने वाली, सबसे पवित्र व महादेव को अपनी भक्ति से प्रसन्न करने वाली मां महागौरी हम सबका भला करो।
The best way to attain Atma Shanti is through contentment and seeing that everything in the world is just a manifestation of the divine truth. There is no second here.