A 14-item, multiple-choice diagnostic assessment tool, the quantum chemistry concept inventory or QCCI, is presented. Items were developed based on published student misconceptions and content coverage and then piloted and used in advanced physical chemistry undergraduate courses. In addition to the instrument itself, data from both a pretest, prior to a semester of instruction, and a post-test, after a semester of instruction, are provided. These data suggest that the QCCI is capable of measuring the variation of student conceptual understanding of quantum mechanics in the context of a physical chemistry course and that the instrument is sensitive to gains in student understanding that result from direct instruction in the topic, even when that instruction has a significant mathematical component as is common for this course.