An ab initio molecular dynamics study on Ng(2)@C-60 (Ng = He-Kr) systems is performed to analyze the movement of Ng(2) molecules inside a C-60 cage. Within 500 fs time window, the He-2 undergoes precession encompassing translation, vibration and rotation readily whereas other Ng(2) molecules show usual vibration but the degrees of translation and rotation decrease with an increase in size of the Ng atoms. Increase in interaction between the Ng centers and cage carbons and an increased distortion of cage in moving from He to Kr seem to be responsible for this. During the movement, the Ng(2) units behave as single entity. (C) 2014 Elsevier B.V. All rights reserved.