closed strings do. In type IIB, the allowed number of p-branes are p = 1 , 3 , 5 , 7 , 9. The two D -branes interact by exchanging gravitons, dilatons and antisymmetric tensors, provided their separation is large compared to the string scale. In a compact manifold the D -branes can modify the dynamics. They do so via gravita- tional field they create, which gives rise to the warping of the metric as shown in Eq. (465). The presence of anti-symmetric tensor fields, for which the branes act as sources also lead to quantization of three-form fluxes similar to the argument for monopoles [43], i.e. where C is a 3-cycle, n$_{1}$, n$_{2}$ are integers. The presence of fluxes induces scalar potential to the moduli, which can be used for fixing them. In this respect fluxes can break some or all the residual SUSY in 4 d . Finally the branes can also wrap around a non-contractable surface, in particularly D 7 branes, which fill 7 spatial dimensions also extends into 4 of the compact 6 dimensions can wrap around 4-cycle in the internal dimensions. The brane tension provides potential for the moduli, depending on how many times it wraps. There are also negative tension branes, known as orientifold planes, i.e. O 7, required to cancel to total charge in the internal dimensions created by the warping of D 7 branes. All these contributions lead to fix all the moduli, which are known as complex structure moduli, within type IIB compactifications [43]. 1 2 πα ′ ∫ c H$_{3}$ ∝ n$_{1}$, 1 2 πα ′ ∫ c F$_{3}$ ∝ n$_{2}$ , (468) In KKLT, flux compactification lead to fixing all the moduli with a constant superpoten- tial term, W$_{0}$ , while the volume modulus, T , is assumed to be fixed by the non-perturbative superpotential term in 4 d [55]: 2. KKLT scenario W ( T ) = W$_{0}$ + $_{Ae}$− aT , K = − 3 ln( T + T ) . (469) where a = 2 π/N, A are constants, K is the K¨ ahler potential, which is assumed to be given at the tree level arising from the compactification volume, K = − 2 ln( M 6 $_{s}$V$_{6}$ ), where M 6 $_{s}$V$_{6}$ ∝ ( T + T ) 3 / $^{2}$. The origin of non-perturbative superpotential term arises due to instanton contribution or the presence of D 7 brane wrapping certain cycles of the internal dimensions carrying asymptotically free, SU ( N ), gauge group. Then the gaugino condensation along 226