Charge bridges

For a mixed even-odd perturbation one has in general AHOMO ALUMO, with either AHOMO > ALUMO or AHOMO < ALUMO. The effects of It substituents and charged bridges with an odd number of centers (e.g., C or C ) can be viewed as a superposition of an even and an odd perturbation, so AHOMO ALUMO in these cases. 

In order to maintain metallic behaviour even at very low temperatures, charge bridges between the one-dimensional stacks are necessary to establish a higher dimensionality. 

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