An atomic-scale modeling and experimental study of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si0029.gif" overflow="scroll"><mml:mrow><mml:mo stretchy="false">〈</mml:mo><mml:mi>c</mml:mi><mml:mo>+</mml:mo><mml:mi>a</mml:mi><mml:mo stretchy="false">〉</mml:mo></mml:mrow></mml:math> dislocations in Mg