The electrochemical factors that control the interaction of a chemisorbed molecule under potential bias are identified by considering the vibrational response of carbon monoxide on a platinum electrode from first principles. Using three complementary approaches to simulate the potential bias, it is shown that the frequency shifts upon electrode bias are electrostatic in nature and give rise to the vibrational Stark effect, in which the first and second-order responses are determined by the dipole moment and the capacitance of the system, respectively. These results are rationalized by examining the central role of electrostatic screening by the metal at the atomic scale. (C) 2014 Elsevier Ltd. All rights reserved.